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Acids, bases, pH and buffers

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OCR AS and A-level Chemistry past-paper questions since 2016 · 69 parts · mark schemes follow the questions · www.chemistryaleveltutor.com · Questions and mark schemes © OCR

1. June 2025 · Paper 1 · Q13 · 1 marks ·
13 A mixture of ethanoic acid and ethanol is left to reach equilibrium. A student determines the amount of ethanoic acid remaining in the equilibrium mixture by titrating the mixture with a solution of sodium hydroxide. Which row of the table shows a suitable indicator for the titration? pH range of the indicator A 1.2 – 2.8 B 4.8 – 6.0 C 9.4 –10.6 D 12.0 –13.4 Your answer [1]
2. June 2025 · Paper 1 · Q19(a) · 1 marks ·
17 16 15 14 13 12 In k 11 10 9 8 7 6 4.0 4.5 5.0 5.5 6.0 6.5 7.0 × 10– 4 × 10– 4 × 10– 4 × 10– 4 × 10– 4 × 10– 4 × 10– 4 1 T / K–1 19 This question is about monobasic acids and buffer solutions. (a) Nitric acid, HNO3, is a strong monobasic acid. Calculate the pH of 0.150 mol dm–3 HNO3. Give your answer to 2 decimal places. pH = ............................. [1]
3. June 2025 · Paper 1 · Q19(b)(i) · 2 marks ·
(b) (i) Nitrous acid, HNO2, is a weak monobasic acid. Predict whether the pH of 0.150 mol dm–3 HNO2 is higher, lower or the same as the pH of 0.150 mol dm–3 nitric acid, HNO3. Explain your answer. Prediction ........................................................ Explanation ...................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... ..................................................................................................................................................... [2]
4. June 2025 · Paper 1 · Q19(c) · 6 marks ·
(c)* A student plans to prepare a buffer solution with a pH of 4.85 using propanoic acid, CH3CH2COOH, and sodium propanoate, CH3CH2COONa. Ka for CH3CH2COOH = 1.32 × 10–5 mol dm–3. The student mixes 125 cm3 of 0.180 mol dm–3 CH3CH2COOH(aq) with a calculated mass of CH3CH2COONa(s). The student assumes that the volume of the solution does not change. • Calculate the mass of CH3CH2COONa needed to make 125 cm3 of a buffer solution with a pH of 4.85. • Explain why this solution resists changes in pH when a small amount of alkali is added. .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... ..................................................................................................................................................... [6] Additional answer space if required. .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... ..........................................................................................................................................................
5. June 2025 · Paper 3 · Q2(c) · 5 marks ·
(c) Barium hydroxide, Ba(OH)2, is an alkali. A student is supplied with a solution of Ba(OH)2 with a concentration of 46.80 g dm–3 at 25 °C. The student dilutes 20.0 cm3 of this solution of Ba(OH)2 with water and the solution is made up to 100.0 cm3 at 25 °C. Calculate the pH of the diluted solution of Ba(OH)2 at 25 °C. Give your answer to 2 decimal places. pH = .......................................................... [5]
6. June 2025 · Paper 3 · Q5(a)(v) · 1 marks ·
(v) Suggest why the student heated the mixture from Step 2 with dilute aqueous acid in Step 3. .......................................................................................................................................................... .......................................................................................................................................................... ..................................................................................................................................................... [1]
7. June 2024 · Paper 1 · Q12 · 1 marks ·
12 Which solution can be added to CH3COOH(aq) to make a buffer solution? A CH3COONa(aq) B HCOOH(aq) C HCl (aq) D NaCl (aq) Your answer [1]
8. June 2024 · Paper 1 · Q19(a)(i) · 1 marks ·
19 This question is about acids and bases. (a) Chloroethanoic acid, Cl CH2COOH, is a weak monobasic acid. (i) Write the expression for the acid dissociation constant, Ka, of Cl CH2COOH. [1]
9. June 2024 · Paper 1 · Q19(a)(ii) · 1 marks ·
(ii) The expression for the acid dissociation constant, Ka, of Cl CH2COOH can be simplified to: [H+]2 Ka = [Cl CH2COOH] Expression 19.1 State one approximation that allows the expression from (a)(i) to be simplified to Expression 19.1. .......................................................................................................................................................... ..................................................................................................................................................... [1]
10. June 2024 · Paper 1 · Q19(a)(iii) · 3 marks ·
(iii) A student carries out an experiment to determine the pKa value of a solution of Cl CH2COOH. • The concentration of Cl CH2COOH is 0.090 mol dm–3. • The pH of Cl CH2COOH is 1.95. Use Expression 19.1 to calculate the pKa value of Cl CH2COOH. Give your answer to 2 decimal places. pKa = .......................................................... [3]
11. June 2024 · Paper 1 · Q19(b)(i) · 5 marks ·
(b) A student titrates a 10.0 cm3 sample of ethanoic acid, CH3COOH, against an aqueous solution of 0.0560 mol dm–3 Ba(OH)2. 2CH3COOH + Ba(OH)2 Ba(CH3COO)2 + 2H2O The student used a pH meter to measure the pH of the mixture after every addition of Ba(OH)2 throughout the titration. The student’s results are shown below. 12 11 10 9 8 7 pH 6 5 4 3 2 1 0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 volume of Ba(OH)2 / cm3 (i) Draw a best-fit curve on the graph and calculate the concentration of the CH3COOH solution. CH3COOH concentration = .......................................... mol dm–3 [5]
12. June 2024 · Paper 1 · Q19(b)(ii) · 1 marks ·
(ii) The end point of the titration can also be found by observing the colour change of an indicator. The pH ranges of some indicators are shown in the table. Indicator pH range Malachite green 0.2 – 1.8 Bromophenol blue 2.8 – 4.6 Phenol red 6.8 – 8.4 Phenolphthalein 8.2 – 10.0 Identify the indicator in the table that would be suitable to observe the end point of the titration between CH3COOH and Ba(OH)2. ..................................................................................................................................................... [1]
13. June 2024 · Paper 1 · Q22(c) · 5 marks ·
(c) Red blood cells contain haemoglobin which transports oxygen around the body. For efficient transportation of oxygen, healthy human blood must be maintained at a pH value between 7.35 and 7.45. Human blood acts as a buffer due to the presence of carbonic acid, H2CO3, and hydrogencarbonate, HCO3 –, ions as shown below. H2CO3(aq) H+(aq) + HCO3 –(aq) Ka = 4.27 × 10–7 mol dm–3 • Explain, using ligand substitution, how haemoglobin transports oxygen around the body. • Determine whether a sample of blood with a [HCO3 –] : [H2CO3] ratio of 8.5:1 is healthy. .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... ..................................................................................................................................................... [5]
14. June 2024 · Paper 3 · Q1(a) · 2 marks ·
1 This question is about acids, bases and salts. (a) What is the pH of 1.00 dm3 of 0.400 mol dm–3 of NaOH(aq) at 298 K? pH = ................... [2]
15. June 2024 · Paper 3 · Q1(b) · 1 marks ·
(b) Water is added to 10.0 cm3 of 0.750 mol dm–3 HCl (aq) to produce 100 cm3 of diluted HCl (aq). What is the pH of the diluted HCl (aq)? Give your answer to 2 decimal places. pH = ................... [1]
16. June 2024 · Paper 3 · Q1(c) · 2 marks ·
(c) A solution has concentrations of 0.300 mol dm–3 CH3COOH(aq) and 0.100 mol dm–3 CH3COONa(aq). Ka for CH3COOH = 1.75 × 10–5 mol dm–3 at 298 K. What is the pH of the solution at 298 K? Give your answer to 2 decimal places. pH = ................... [2]
17. June 2024 · Paper 3 · Q4(a)(i) · 2 marks ·
4 This question is about the chemistry of compounds containing phosphorus. (a) Phosphorus forms several acids including H3PO4 and H3PO3. H3PO4 is a tribasic acid. The equilibria for the dissociations are shown below. 1 H3PO4 H+ + H2PO4 – 2 H2PO4 – H+ + HPO4 2– 3 HPO4 2– H+ + PO4 3– (i) During the equilibria, H2PO4 – behaves both as an acid and as a base. Explain this statement, using the equilibria 1, 2 and 3, as required. .......................................................................................................................................................... .......................................................................................................................................................... .......................................................................................................................................................... ..................................................................................................................................................... [2]
18. June 2023 · Paper 1 · Q10 · 1 marks · AO2
10 What is the percentage dissociation of a 0.015 mol dm–3 solution of methanoic acid, HCOOH (Ka = 1.60 × 10–4 mol dm–3)? A 0.016% B 1.1% C 1.82% D 10.3% Your answer [1]
19. June 2023 · Paper 1 · Q20(a)(i) · 2 marks · AO1, 2
20 This question is about acids and bases. (a) Table 20.1 shows the ionic product, Kw, of water at 25 °C and 40 °C. Table 20.1 Temperature / °C Kw / mol2 dm–6 25 1.00 × 10−14 40 2.92 × 10−14 (i) Calculate the pH of water at 40 °C. Give your answer to 2 decimal places. pH = .......................................................... [2]
20. June 2023 · Paper 1 · Q20(a)(ii) · 1 marks · AO3
(ii) Table 20.1 shows different Kw values at 25 °C and at 40 °C. A student suggests that water is neutral at these temperatures. Explain why this student is correct. ........................................................................................................................................... ........................................................................................................................................... ...................................................................................................................................... [1]
21. June 2023 · Paper 1 · Q20(b) · 5 marks · AO1, 2
(b) A student reacts strontium metal with water to make a 250.0 cm3 solution of aqueous strontium hydroxide, Sr(OH)2. The solution contains 0.145 g of strontium hydroxide. • Write an equation for the reaction of strontium with water. • Calculate the pH of this 250.0 cm3 solution of strontium hydroxide at 40 °C. You should refer back to Table 20.1 at the start of (a). Give your answer to 2 decimal places. Equation ................................................................................................................................... Calculation pH = .......................................................... [5]
22. June 2023 · Paper 1 · Q20(d)(i) · 1 marks · AO2
(d) A student reacts an excess of magnesium with 25.0 cm3 of 0.500 mol dm–3 hydrochloric acid, HCl . The student also reacts an excess of magnesium with 25.0 cm3 of 0.500 mol dm–3 ethanoic acid, CH3COOH. (i) Construct an ionic equation for the reaction of magnesium with an acid. ...................................................................................................................................... [1]
23. June 2023 · Paper 1 · Q20(d)(ii) · 3 marks · AO1, 3
(ii) Explain why these two reactions of magnesium produce the same volume of gas but at different rates. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ...................................................................................................................................... [3]
24. June 2023 · Paper 1 · Q20(e)(i) · 1 marks · AO1
(e) Butanoic acid, CH3CH2CH2COOH, is a weak monobasic acid. (i) Explain what is meant by the term monobasic acid. ........................................................................................................................................... ...................................................................................................................................... [1]
25. June 2023 · Paper 1 · Q20(e)(ii) · 4 marks · AO1, 2
(ii) A buffer solution is prepared by dissolving 3.39 g of potassium hydroxide in 250 cm3 of 0.376 mol dm–3 butanoic acid. This buffer solution has a pH of 5.07 at 25 °C. Calculate the acid dissociation constant, Ka, of butanoic acid at 25 °C. Assume that the volume of the solution remains constant at 250 cm3 when the potassium hydroxide is dissolved. Ka = .......................................... mol dm–3 [4]
26. June 2023 · Paper 1 · Q20(f) · 1 marks · AO3
(f) A buffer solution has a pH of 4.50. When a small volume of water is added to this buffer solution, the pH does not change. Explain why the pH does not change. ................................................................................................................................................... ................................................................................................................................................... .............................................................................................................................................. [1]
27. June 2023 · Paper 3 · Q2(b) · 3 marks · AO3
(b) A student investigates the rate of a reaction that is 1st order with respect to hydrochloric acid, HCl (aq). • The student carries out a reaction using 0.680 mol dm–3 HCl (aq). The initial rate is 9.52 × 10–4 mol dm–3 s–1. • The student dilutes a different sample of 0.680 mol dm–3 HCl (aq) with water. The pH of this diluted acid is 1.50. • The student repeats the reaction using the same volume of this diluted acid. Determine the initial rate using this diluted acid. initial rate = ..................................... mol dm–3 s–1 [3]
28. June 2022 · Paper 1 · Q11 · 1 marks · AO2
11 The equilibrium equation for an indicator, HA, is shown below. Equation: HA(aq) A–(aq) + H+(aq) Colour: Blue Yellow The indicator is added to a solution. The indicator turns a yellow colour. An excess of aqueous sodium hydroxide is then added. Which statement describes how the colour of this solution would be expected to change? A Colour changes from yellow to blue. B Colour changes from yellow to green. C Colour changes from yellow to green and then to blue. D Colour stays yellow. Your answer [1]
29. June 2022 · Paper 1 · Q12 · 1 marks · AO1
12 Ammonia and water react to set up an acid–base equilibrium. What are the Brønsted–Lowry acids in the equilibrium mixture? A H2O and OH– B OH– and NH3 C NH4 + and H2O D NH4 + and NH3 Your answer [1]
30. June 2022 · Paper 1 · Q19(b)(i) · 1 marks · AO1
(b) Glycolic acid, HOCH2COOH, (pKa = 3.83) is a weak monobasic acid used in some skincare products. A buffer solution is prepared by adding 60.0 cm3 of 0.750 mol dm–3 glycolic acid to 40.0 cm3 of 0.625 mol dm–3 potassium hydroxide, KOH. (i) Explain why a buffer solution is formed. ........................................................................................................................................... ........................................................................................................................................... ...................................................................................................................................... [1]
31. June 2022 · Paper 1 · Q19(b)(ii) · 4 marks · AO1, 2
(ii) Calculate the pH of the buffer solution that has been prepared. Give your answer to 2 decimal places. pH = ......................................................... [4]
32. June 2022 · Paper 1 · Q19(b)(iii) · 2 marks · AO1
(iii) A small amount of aqueous ammonia, NH3(aq), is added to the buffer solution. Explain, in terms of equilibrium, how the buffer solution would respond to the added NH3(aq). ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ...................................................................................................................................... [2]
33. June 2022 · Paper 1 · Q21(b)(i) · 5 marks · AO2
(b) A sample of barium oxide is added to distilled water at 25 °C. A colourless solution forms containing barium hydroxide, Ba(OH)2. The solution is made up to 250.0 cm3 with distilled water. The pH of this solution is 13.12. (i) Determine the mass of barium oxide that was used. Give your answer to 3 significant figures. mass of barium oxide = ....................................................... g [5]
34. June 2022 · Paper 3 · Q6(c)(i) · 2 marks · AO3
(c) Vitamin C, C6H8O6, is a weak acid (Ka = 7.94 x 10–5 (mol dm–3)), which is often referred to as ascorbic acid. (i) In aqueous solution, vitamin C donates a proton to water: C6H8O6 + H2O C6H7O6 – + H3O+ Add curly arrows to the diagram to suggest the mechanism for this process. H2O: O O H O O H H H H + H3O+ O O – O O O O O O H H [2]
35. June 2022 · Paper 3 · Q6(c)(ii) · 3 marks · AO1, 2
(ii) The student dissolves 0.150 mol of vitamin C in water and makes the solution up to 250 cm3 in a volumetric flask. Calculate the pH of this solution of vitamin C. Give your answer to 2 decimal places. pH = ......................................................... [3]
36. November 2021 · Paper 1 · Q10 · 1 marks ·
10 An aqueous solution of ethanoic acid, CH3COOH, has a concentration of 0.50 mol dm–3. pKa for CH3COOH = 4.76 at 25 °C. What is the pH of the ethanoic acid solution at 25 °C? A 2.53 B 2.68 C 4.91 D 5.06 Your answer [1]
37. November 2021 · Paper 1 · Q22(a) · 2 marks · AO2
22 This question is about acids, bases and buffers. (a) Sodium hydroxide, NaOH, is a strong base. Calculate the pH of 0.140 mol dm–3 NaOH(aq) at 298 K. Give your answer to 2 decimal places. pH = ......................................................... [2]
38. November 2021 · Paper 1 · Q22(c) · 6 marks · AO1, 2, 3
(c)* Nitrous acid, HNO2, is a weak Brønsted–Lowry acid with a pKa value of 3.34 at room temperature. HNO2 can be prepared by reacting N2O3 with water. HNO2 is the only product. A chemist makes up a buffer solution by the following method. Step 1 The chemist weighs a sample of N2O3. Water is then added to form 100 cm3 of 0.500 mol dm–3 HNO2(aq). Step 2 The chemist adds 100 cm3 of 0.150 mol dm–3 NaOH(aq) to the 100 cm3 solution of 0.500 mol dm–3 solution of HNO2(aq). The resulting solution is made up to 1.00 dm3. Explain why a buffer solution forms in Step 2. Determine the pH of this buffer solution and the mass of N2O3 that was used in Step 1. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... .............................................................................................................................................. [6] Additional answer space if required. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ...................................................................................................................................................
39. November 2021 · Paper 3 · Q5(b)(i) · 2 marks · AO2
(b) Hydrazoic acid, HN3, is a weak acid (Ka = 2.51 × 10–5 mol dm–3). (i) Calculate the pH of 0.125 mol dm–3 hydrazoic acid. Give your answer to 2 decimal places. pH = .......................................................... [2]
40. November 2021 · Paper 3 · Q5(b)(ii) · 2 marks · AO1
(ii) When added to water, hydrazoic acid forms an equilibrium mixture containing conjugate acid–base pairs. Complete the equation for this equilibrium and label the conjugate acid–base pairs as: A1, B1 and A2, B2. Equation HN3 + .................... .................... + .................... Acid-base pairs .................... .................... .................... .................... [2]
41. November 2020 · Paper 1 · Q11 · 1 marks · AO2
11 20 cm3 of 0.10 mol dm−3 hydrochloric acid is added to 10 cm3 of 0.10 mol dm−3 sodium hydroxide. What is the pH of the resulting mixture? A 1.00 B 1.18 C 1.30 D 1.48 Your answer [1]
42. November 2020 · Paper 1 · Q18(a) · 3 marks · AO1, 2
18 This question is about reactions and uses of the weak acids methanoic acid, HCOOH, and ethanoic acid, CH3COOH. (a) A student adds magnesium metal to an aqueous solution of ethanoic acid, CH3COOH. A redox reaction takes place. Write the overall equation for this reaction and explain, in terms of oxidation numbers, which element has been oxidised and which element has been reduced. Equation ................................................................................................................................... Oxidation .................................................................................................................................. ................................................................................................................................................... Reduction ................................................................................................................................. ................................................................................................................................................... [3]
43. November 2020 · Paper 1 · Q18(b) · 2 marks · AO1
(b) The Ka values of HCOOH and CH3COOH are shown in Table 18.1. Weak acid Ka / mol dm−3 HCOOH 1.82 × 10−4 CH3COOH 1.78 × 10−5 Table 18.1 A student adds methanoic acid to ethanoic acid. An equilibrium is set up containing two acid-base pairs. Complete the equilibrium and label the conjugate acid-base pairs as A1, B1 and A2, B2. HCOOH + CH3COOH ..................................... + ..................................... .................... .................... .................... .................... [2]
44. November 2020 · Paper 1 · Q18(c)(i) · 2 marks · AO2
(c) Use Table 18.1 to answer the following questions. (i) The student measures the pH of CH3COOH(aq) as 2.72. Show that the concentration of the CH3COOH(aq) is 0.204 mol dm−3. [2]
45. November 2020 · Paper 1 · Q18(c)(ii) · 4 marks · AO3
(ii) The student plans to make a buffer solution of pH 4.00 from a mixture of CH3COOH(aq) and sodium ethanoate, CH3COONa(aq). The student mixes 400 cm3 of 0.204 mol dm−3 CH3COOH(aq) with 600 cm3 of CH3COONa(aq). Calculate the concentration of CH3COONa(aq) needed to prepare this buffer solution of pH 4.00. concentration = .......................................... mol dm−3 [4]
46. November 2020 · Paper 3 · Q6(b) · 4 marks · AO2, 3
(b) Ethanoic acid, CH3COOH, and methanesulfonic acid, CH3SO2OH, are both monobasic acids. The pKa values are shown in the table. Acid pKa Ethanoic acid CH3COOH 4.76 Methanesulfonic acid CH3SO2OH –1.90 A student suggests that 1.0 mol dm−3 CH3SO2OH should have a lower pH value than 1.0 mol dm−3 CH3COOH. Write an equation, showing conjugate acid–base pairs, for the equilibrium of CH3SO2OH with water and explain, with reasons, whether the student is correct. Label the conjugate acid–base pairs: A1, B1 and A2, B2. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... .............................................................................................................................................. [4]
47. June 2019 · Paper 1 · Q12 · 1 marks · AO1
12 The equation shows the dissociation of the acid H3AsO4 in water. H3AsO4 + H2O H2AsO4 – + H3O+ Which pair is a conjugate acid–base pair? A H3AsO4 and H2O B H2AsO4 − and H3O+ C H3AsO4 and H3O+ D H3O+ and H2O Your answer [1]
48. June 2019 · Paper 1 · Q17(a) · 6 marks · AO1, 3
17 Healthy human blood needs to be maintained at a pH of 7.40 for the body to function normally. (a)* Carbonic acid, H2CO3, is a weak acid which, together with hydrogencarbonate ions, HCO3 −, acts as a buffer to maintain the pH of blood. The pKa value for the dissociation of carbonic acid is 6.38. Explain, in terms of equilibrium, how the carbonic acid–hydrogencarbonate mixture acts as a buffer in the control of blood pH, and calculate the [HCO3 −] : [H2CO3] ratio in healthy blood. [6] ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... Additional answer space if required ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ...................................................................................................................................................
49. June 2019 · Paper 1 · Q20(a) · 2 marks · AO2
20 This question is about weak acids. The Ka values of three weak acids are shown in Table 20.1. Weak acid Ka / mol dm−3 iodic(V) acid, HIO3(aq) 1.78 × 10−1 propanoic acid, C2H5COOH(aq) 1.35  ×  10−5 hydrocyanic acid, HCN(aq) 6.17 × 10−10 Table 20.1 (a) Calculate the pH of 0.0800 mol dm−3 C2H5COOH(aq). Give your answer to 2 decimal places. pH = ......................................................... [2]
50. June 2019 · Paper 1 · Q20(b)(ii) · 4 marks · AO1, 2
(ii) Calculate the pH of the final solution. Give your answer to 2 decimal places. pH = ......................................................... [4]
51. June 2019 · Paper 1 · Q20(b)(iii) · 3 marks · AO2
(iii) On the axes below, sketch a pH curve for the pH changes during the addition of 45.0 cm3 of 0.100 mol dm−3 NaOH(aq) to 25.0 cm3 of 0.0800 mol dm−3 C2H5COOH(aq). 14 pH 7 0 0 10 20 30 40 50 volume NaOH(aq) added / cm3 [3]
52. June 2019 · Paper 1 · Q20(b)(iv) · 1 marks · AO3
(iv) The student considers using the four indicators in Table 20.2 for the titration. Indicator pH range Cresol red 0.2 – 1.8 Bromophenol blue 3.0 – 4.6 Cresol purple 7.6 – 9.2 Indigo carmine 11.6 – 14.0 Table 20.2 Explain which indicator would be most suitable for the titration. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ...................................................................................................................................... [1]
53. June 2019 · Paper 1 · Q20(b)(v) · 2 marks · AO3
(v) The student repeats the experiment starting with 25.0 cm3 of 0.0800 mol dm−3 HCN(aq) and adding a total of 45.0 cm3 of 0.100 mol dm−3 NaOH(aq). Predict one similarity and one difference between the pH curve with C2H5COOH(aq) and the pH curve with HCN(aq). Use the information in Table 20.1, and your answer to (b)(iii). Similarity ............................................................................................................................ ........................................................................................................................................... Difference .......................................................................................................................... ........................................................................................................................................... [2]
54. June 2019 · Paper 1 · Q20(c) · 1 marks · AO3
(c) The student calculates the pH of 0.0800 mol dm−3 HIO3(aq). The student assumes that the equilibrium concentration of HIO3(aq) is the same as the initial concentration of HIO3(aq). The student measures the pH, and finds that the measured pH value is different from the calculated pH value. Explain why the measured pH is different from the calculated pH. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... .............................................................................................................................................. [1]
55. June 2019 · Paper 3 · Q1(e) · 1 marks · AO2
(e) A reaction is first order with respect to H+. At a pH of 1, the initial rate is 2.4 × 10−3 mol dm−3 s−1. What is the initial rate at a pH of 3? initial rate = ..................................... mol dm−3 s−1 [1]
56. June 2019 · Paper 3 · Q2(a) · 1 marks · AO1
2 Benzoic acid, C6H5COOH, is added to some foods as a preservative. A student prepares benzoic acid as outlined below. Step 1 The student mixes 4.00 cm3 of phenylmethanol, C6H5CH2OH, (density = 1.04 g cm−3) with sodium carbonate and aqueous potassium manganate(VII), as an oxidising agent. The mixture is heated under reflux. Step 2 The resulting mixture is cooled and then acidified with concentrated HCl . Impure crystals of benzoic acid appear. Step 3 The student recrystallises the impure crystals to obtain 1.59 g of pure benzoic acid. (a) In Step 1, sodium carbonate, Na2CO3, makes the reaction mixture alkaline. Write an ionic equation to show how carbonate ions form an alkaline solution in water. .............................................................................................................................................. [1]
57. June 2019 · Paper 3 · Q2(b) · 1 marks · AO2
(b) In Step 2, explain why the mixture must be acidified so that crystals of benzoic acid appear. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... .............................................................................................................................................. [1]
58. June 2019 · Paper 3 · Q4(a)(iv) · 5 marks · AO1, 2
(iv) A student measures the pH of the contents in a bottle of Dettol® as 5.14. The label on the bottle shows the percentage of chloroxylenol in Dettol® as 4.80% i.e. 100 cm3 of Dettol® contains 4.80 g of chloroxylenol. Assume the following: • Chloroxylenol is the only acidic component in Dettol®. • Chloroxylenol is a weak monobasic acid. • The density of Dettol® is 1.00 g cm−3. Write the equation, using molecular formulae, for the acid dissociation of chloroxylenol. Calculate the acid dissociation constant, Ka, for chloroxylenol. Ka = .......................................... mol dm−3 [5]
59. June 2018 · Paper 1 · Q10 · 1 marks · AO2
10 A buffer solution is prepared by mixing 200 cm3 of 2.00 mol dm–3 propanoic acid, CH3CH2COOH, with 600 cm3 of 1.00 mol dm–3 sodium propanoate, CH3CH2COONa. Ka for CH3CH2COOH = 1.32 × 10–5 mol dm–3 What is the pH of the buffer solution? A 4.58 B 4.70 C 5.06 D 5.18 Your answer [1]
60. June 2018 · Paper 1 · Q19(a)(i) · 1 marks ·
19 This question is about acids and bases found in the home. (a) Ethanoic acid, CH3COOH, is the acid present in vinegar. A student carries out an experiment to determine the pKa value of CH3COOH. • The concentration of CH3COOH in the vinegar is 0.870 mol dm–3. • The pH of the vinegar is 2.41. (i) Write the expression for the acid dissociation constant, Ka, of CH3COOH. [1]
61. June 2018 · Paper 1 · Q19(a)(ii) · 3 marks ·
(ii) Calculate the pKa value of CH3COOH. Give your answer to two decimal places. pKa = ......................................................... [3]
62. June 2018 · Paper 1 · Q19(a)(iii) · 1 marks ·
(iii) Determine the percentage dissociation of ethanoic acid in the vinegar. Give your answer to three significant figures. percentage dissociation = ..................................................... % [1]
63. June 2018 · Paper 1 · Q19(b) · 4 marks ·
(b) Many solid drain cleaners are based on sodium hydroxide, NaOH. • A student dissolves 1.26 g of a drain cleaner in water and makes up the solution to 100.0 cm3. • The student measures the pH of this solution as 13.48. Determine the percentage, by mass, of NaOH in the drain cleaner. Give your answer to three significant figures. percentage = ..................................................... % [4]
64. June 2018 · Paper 3 · Q6(a)(iii) · 3 marks ·
(iii) Reaction 6.1 takes place in two steps. OH– ions act as a catalyst. In step 1, ethanal reacts with OH– ions to set up an acid–base equilibrium. In step 2, compound A is formed. • Complete the equilibrium for step 1 and label the conjugate acid–base pairs as: A1, B1 and A2, B2. CH3CHO + OH– .......................... + .......................... ................. ................. ................. ................. • Suggest the equation for step 2. [3]
65. June 2017 · Paper 1 · Q21(a) · 3 marks ·
21 This question is about the properties and reactions of ethanoic acid, CH3COOH. Ethanoic acid is a weak acid with an acid dissociation constant, Ka, of 1.75 × 10–5 mol dm–3 at 25 °C. (a) A student uses a pH meter to measure the pH of a solution of CH3COOH at 25 °C. The measured pH is 2.440. Calculate the concentration of ethanoic acid in the solution. Give your answer to three significant figures. concentration = .......................................... mol dm–3 [3]
66. June 2017 · Paper 1 · Q21(b) · 2 marks ·
(b) Ethanoic acid is added to another weak acid, fluoroethanoic acid, FCH2COOH (Ka = 2.19 × 10–3 mol dm–3). An equilibrium is set up containing two acid-base pairs. Complete the equilibrium and label the conjugate acid-base pairs as A1, B1 and A2, B2. CH3COOH + FCH2COOH ...................................... + ...................................... .............. .............. .............. .............. [2]
67. June 2017 · Paper 1 · Q21(c)(i) · 5 marks ·
(c) The student plans to prepare a buffer solution that has a pH of 4.50. The buffer solution will contain ethanoic acid, CH3COOH, and sodium ethanoate, CH3COONa. The student plans to add 9.08 g CH3COONa to 250 cm3 of 0.800 mol dm–3 CH3COOH. The student assumes that the volume of the solution does not change. (i) Show by calculation whether, or not, the student’s experimental method would produce the required pH. Show all your working. [5]
68. June 2017 · Paper 1 · Q21(c)(ii) · 2 marks ·
(ii) When the student prepares the buffer solution, the volume of solution increases slightly. Suggest whether the pH of the buffer solution would be the same, greater than, or less than your calculated value in (c)(i). Explain your reasoning. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ...................................................................................................................................... [2]
69. June 2017 · Paper 3 · Q5(a)(i) · 1 marks ·
5 This question is about organic molecules that have a strong smell. (a) Thiols are foul-smelling, organic sulfur compounds with the functional group –SH. Butane-1-thiol, shown below, contributes to the strong smell of skunks. SH butane-1-thiol (i) Thiols are weak acids. Write the expression for the acid dissociation constant, Ka, for butane-1-thiol. [1]

Mark schemes

1. June 2025 · Paper 1 · Q13
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2. June 2025 · Paper 1 · Q19(a)
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3. June 2025 · Paper 1 · Q19(b)(i)
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4. June 2025 · Paper 1 · Q19(c)
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6. June 2025 · Paper 3 · Q5(a)(v)
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7. June 2024 · Paper 1 · Q12
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8. June 2024 · Paper 1 · Q19(a)(i)
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9. June 2024 · Paper 1 · Q19(a)(ii)
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10. June 2024 · Paper 1 · Q19(a)(iii)
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11. June 2024 · Paper 1 · Q19(b)(i)
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12. June 2024 · Paper 1 · Q19(b)(ii)
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13. June 2024 · Paper 1 · Q22(c)
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14. June 2024 · Paper 3 · Q1(a)
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15. June 2024 · Paper 3 · Q1(b)
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16. June 2024 · Paper 3 · Q1(c)
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17. June 2024 · Paper 3 · Q4(a)(i)
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18. June 2023 · Paper 1 · Q10
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19. June 2023 · Paper 1 · Q20(a)(i)
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20. June 2023 · Paper 1 · Q20(a)(ii)
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21. June 2023 · Paper 1 · Q20(b)
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22. June 2023 · Paper 1 · Q20(d)(i)
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23. June 2023 · Paper 1 · Q20(d)(ii)
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24. June 2023 · Paper 1 · Q20(e)(i)
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25. June 2023 · Paper 1 · Q20(e)(ii)
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26. June 2023 · Paper 1 · Q20(f)
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27. June 2023 · Paper 3 · Q2(b)
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28. June 2022 · Paper 1 · Q11
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29. June 2022 · Paper 1 · Q12
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30. June 2022 · Paper 1 · Q19(b)(i)
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31. June 2022 · Paper 1 · Q19(b)(ii)
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32. June 2022 · Paper 1 · Q19(b)(iii)
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33. June 2022 · Paper 1 · Q21(b)(i)
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34. June 2022 · Paper 3 · Q6(c)(i)
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35. June 2022 · Paper 3 · Q6(c)(ii)
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36. November 2021 · Paper 1 · Q10
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37. November 2021 · Paper 1 · Q22(a)
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38. November 2021 · Paper 1 · Q22(c)
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39. November 2021 · Paper 3 · Q5(b)(i)
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40. November 2021 · Paper 3 · Q5(b)(ii)
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41. November 2020 · Paper 1 · Q11
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42. November 2020 · Paper 1 · Q18(a)
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43. November 2020 · Paper 1 · Q18(b)
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44. November 2020 · Paper 1 · Q18(c)(i)
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45. November 2020 · Paper 1 · Q18(c)(ii)
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46. November 2020 · Paper 3 · Q6(b)
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47. June 2019 · Paper 1 · Q12
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48. June 2019 · Paper 1 · Q17(a)
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49. June 2019 · Paper 1 · Q20(a)
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50. June 2019 · Paper 1 · Q20(b)(ii)
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51. June 2019 · Paper 1 · Q20(b)(iii)
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52. June 2019 · Paper 1 · Q20(b)(iv)
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53. June 2019 · Paper 1 · Q20(b)(v)
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54. June 2019 · Paper 1 · Q20(c)
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55. June 2019 · Paper 3 · Q1(e)
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56. June 2019 · Paper 3 · Q2(a)
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57. June 2019 · Paper 3 · Q2(b)
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58. June 2019 · Paper 3 · Q4(a)(iv)
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59. June 2018 · Paper 1 · Q10
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60. June 2018 · Paper 1 · Q19(a)(i)
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61. June 2018 · Paper 1 · Q19(a)(ii)
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62. June 2018 · Paper 1 · Q19(a)(iii)
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63. June 2018 · Paper 1 · Q19(b)
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64. June 2018 · Paper 3 · Q6(a)(iii)
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65. June 2017 · Paper 1 · Q21(a)
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66. June 2017 · Paper 1 · Q21(b)
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67. June 2017 · Paper 1 · Q21(c)(i)
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68. June 2017 · Paper 1 · Q21(c)(ii)
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69. June 2017 · Paper 3 · Q5(a)(i)
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