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Equilibria (acid–base): OCR A‑Level Chemistry B questions

Every OCR A-Level Chemistry B exam question on O Equilibria (acid–base) (specification points O(i)–(m)), 2016–2025, with its mark scheme and examiner’s report.

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2,049 questions

  1. Q34(a)(i)

    June 2025 · Paper 1

    1 markO Equilibria (acid–base)
    The whole paper ↗
    June 2025 Paper 1, question 34(a)(i): Explain why this equation shows that H2PO4 [1]

  2. Q34(a)(ii)

    June 2025 · Paper 1

    1 markO Equilibria (acid–base)
    The whole paper ↗
    June 2025 Paper 1, question 34(a)(ii): Write the formula of the conjugate acid of H2PO4 [1]

  3. Q34(a)(iii)

    June 2025 · Paper 1

    4 marksO Equilibria (acid–base)
    The whole paper ↗
    June 2025 Paper 1, question 34(a)(iii): Explain how this buffer works. [4]

  4. Q34(a)(iv)

    June 2025 · Paper 1

    5 marksO Equilibria (acid–base)
    The whole paper ↗
    June 2025 Paper 1, question 34(a)(iv): This type of lateral flow test only works at a pH around 7.4. A scientist wants to make a buffer of pH 7.40, using NaH2PO4 and Na2HPO4. – 2– + H+ pKa = 7.20 H2P

  5. Q34(b)(i)

    June 2025 · Paper 1

    1 markO Equilibria (acid–base)
    The whole paper ↗
    June 2025 Paper 1, question 34(b)(i): Complete the equation for the reaction of hydrochloric acid with water. HCl + H2O [1]

  6. Q34(b)(ii)

    June 2025 · Paper 1

    1 markO Equilibria (acid–base)
    Mark scheme (PDF)
    The whole paper ↗
    June 2025 Paper 1, question 34(b)(ii): Calculate the pH of 0.050 mol dm–3 hydrochloric acid. pH = [1]

  7. Q4(a)(i)

    June 2024 · Paper 2

    1 markO Equilibria (acid–base)
    The whole paper ↗
    June 2024 Paper 2, question 4(a)(i): Write the expression for Ka. Ka = [1]

  8. Q4(a)(ii)

    June 2024 · Paper 2

    2 marksO Equilibria (acid–base)
    The whole paper ↗
    June 2024 Paper 2, question 4(a)(ii): Calculate the pH of a 0.15 mol dm–3 solution of CH3COOH. pH = [2]

  9. Q4(a)(iii)

    June 2024 · Paper 2

    2 marksO Equilibria (acid–base)
    The whole paper ↗
    June 2024 Paper 2, question 4(a)(iii): A student says that the pH would be higher if the ethanoic acid was stronger. Comment on this statement. [2]

  10. Q4(b)

    June 2024 · Paper 2

    2 marksO Equilibria (acid–base)
    The whole paper ↗
    June 2024 Paper 2, question 4(b): The students carry out a reaction between a 0.15 mol dm–3 solution of sodium hydroxide and a 0.15 mol dm–3 solution of ethanoic acid. • Student 1 says that equa

  11. Q4(c)

    June 2024 · Paper 2

    3 marksO Equilibria (acid–base)
    The whole paper ↗
    June 2024 Paper 2, question 4(c): For the salt of a weak acid and a strong base, [OH–]2 = [salt] × Kw /Ka Use this formula to work out the pH of a 0.075 mol dm–3 sodium ethanoate solution. (The

  12. Q4(d)(i)

    June 2024 · Paper 2

    2 marksO Equilibria (acid–base)
    The whole paper ↗
    June 2024 Paper 2, question 4(d)(i): [H+] = mol dm–3 [2]

  13. Q4(d)(ii)

    June 2024 · Paper 2

    2 marksO Equilibria (acid–base)
    The whole paper ↗
    June 2024 Paper 2, question 4(d)(ii): The student adds a small amount of acid to this buffer. Explain why the pH changes very little. CH3COO– + H+ Use Equation 4.1: CH3COOH [2]

  14. Q33(a)(ii)

    June 2023 · Paper 1

    2 marksO Equilibria (acid–base)
    The whole paper ↗
    June 2023 Paper 1, question 33(a)(ii): The pH of the sodium hydroxide solution used in this salt preparation is 13.46 at 25 °C. Calculate the concentration of the sodium hydroxide solution. concentra

  15. Q33(b)(i)

    June 2023 · Paper 1

    1 markO Equilibria (acid–base)
    The whole paper ↗
    June 2023 Paper 1, question 33(b)(i): Identify the Brønsted-Lowry acid and conjugate base in Equation 33.2. Brønsted-Lowry acid Conjugate base [1]

  16. Q33(b)(ii)

    June 2023 · Paper 1

    4 marksO Equilibria (acid–base)
    The whole paper ↗
    June 2023 Paper 1, question 33(b)(ii): Using Equation 33.2, explain how this buffer solution maintains its pH when small amounts of acid and alkali are added. [4]

  17. Q33(b)(iii)

    June 2023 · Paper 1

    4 marksO Equilibria (acid–base)
    The whole paper ↗
    June 2023 Paper 1, question 33(b)(iii): At body temperature, healthy blood at a pH of 7.40 has a concentration ratio – HCO3 = 11.3 H2CO3 If the pH falls below 7.35, a condition known as acidosis may d

  18. Q6

    June 2022 · Paper 1

    1 markO Equilibria (acid–base)
    The whole paper ↗
    June 2022 Paper 1, question 6: Which mixture forms a buffer solution? A A mixture of a strong acid and a strong base B A mixture of a strong acid with its salt C A mixture of a strong base wi

  19. Q19

    June 2022 · Paper 1

    1 markO Equilibria (acid–base)
    The whole paper ↗
    June 2022 Paper 1, question 19: What is correct about a 0.1 mol dm–3 solution of a strong base? [OH–] << 0.1 mol dm–3. A The solution is weaker than a 0.5 mol dm–3 solution of the same base. B

  20. Q21

    June 2022 · Paper 1

    1 markO Equilibria (acid–base)
    The whole paper ↗
    June 2022 Paper 1, question 21: In the absence of water the following reaction occurs. + + HSO4 – CH3COOH + H2SO4 CH3C(OH)2 What is correct about this reaction? A CH3COOH is a proton donor. B
All 62 questions on Equilibria (acid–base), as a list
  1. June 2025, Paper 1, Q34(a)(i) (1 mark): Explain why this equation shows that H2PO4 [1]
  2. June 2025, Paper 1, Q34(a)(ii) (1 mark): Write the formula of the conjugate acid of H2PO4 [1]
  3. June 2025, Paper 1, Q34(a)(iii) (4 marks): Explain how this buffer works. [4]
  4. June 2025, Paper 1, Q34(a)(iv) (5 marks): This type of lateral flow test only works at a pH around 7.4. A scientist wants to make a buffer of pH 7.40, using NaH2PO4 and Na2HPO4. – 2– + H+ pKa = 7.20 H2P
  5. June 2025, Paper 1, Q34(b)(i) (1 mark): Complete the equation for the reaction of hydrochloric acid with water. HCl + H2O [1]
  6. June 2025, Paper 1, Q34(b)(ii) (1 mark): Calculate the pH of 0.050 mol dm–3 hydrochloric acid. pH = [1]
  7. June 2024, Paper 2, Q4(a)(i) (1 mark): Write the expression for Ka. Ka = [1]
  8. June 2024, Paper 2, Q4(a)(ii) (2 marks): Calculate the pH of a 0.15 mol dm–3 solution of CH3COOH. pH = [2]
  9. June 2024, Paper 2, Q4(a)(iii) (2 marks): A student says that the pH would be higher if the ethanoic acid was stronger. Comment on this statement. [2]
  10. June 2024, Paper 2, Q4(b) (2 marks): The students carry out a reaction between a 0.15 mol dm–3 solution of sodium hydroxide and a 0.15 mol dm–3 solution of ethanoic acid. • Student 1 says that equa
  11. June 2024, Paper 2, Q4(c) (3 marks): For the salt of a weak acid and a strong base, [OH–]2 = [salt] × Kw /Ka Use this formula to work out the pH of a 0.075 mol dm–3 sodium ethanoate solution. (The
  12. June 2024, Paper 2, Q4(d)(i) (2 marks): [H+] = mol dm–3 [2]
  13. June 2024, Paper 2, Q4(d)(ii) (2 marks): The student adds a small amount of acid to this buffer. Explain why the pH changes very little. CH3COO– + H+ Use Equation 4.1: CH3COOH [2]
  14. June 2023, Paper 1, Q33(a)(ii) (2 marks): The pH of the sodium hydroxide solution used in this salt preparation is 13.46 at 25 °C. Calculate the concentration of the sodium hydroxide solution. concentra
  15. June 2023, Paper 1, Q33(b)(i) (1 mark): Identify the Brønsted-Lowry acid and conjugate base in Equation 33.2. Brønsted-Lowry acid Conjugate base [1]
  16. June 2023, Paper 1, Q33(b)(ii) (4 marks): Using Equation 33.2, explain how this buffer solution maintains its pH when small amounts of acid and alkali are added. [4]
  17. June 2023, Paper 1, Q33(b)(iii) (4 marks): At body temperature, healthy blood at a pH of 7.40 has a concentration ratio – HCO3 = 11.3 H2CO3 If the pH falls below 7.35, a condition known as acidosis may d
  18. June 2022, Paper 1, Q6 (1 mark): Which mixture forms a buffer solution? A A mixture of a strong acid and a strong base B A mixture of a strong acid with its salt C A mixture of a strong base wi
  19. June 2022, Paper 1, Q19 (1 mark): What is correct about a 0.1 mol dm–3 solution of a strong base? [OH–] << 0.1 mol dm–3. A The solution is weaker than a 0.5 mol dm–3 solution of the same base. B
  20. June 2022, Paper 1, Q21 (1 mark): In the absence of water the following reaction occurs. + + HSO4 – CH3COOH + H2SO4 CH3C(OH)2 What is correct about this reaction? A CH3COOH is a proton donor. B
  21. June 2022, Paper 1, Q22 (1 mark): What is correct? A 0.10 mol dm–3 solution of a strong acid has a pH of 2. A A 0.10 mol dm–3 solution of a strong base has a pH of 14. B A 0.10 mol dm–3 solution
  22. June 2022, Paper 1, Q23 (1 mark): 10 cm3 of 0.10 mol dm–3 HCl (aq) is added to 5 cm3 of 0.10 mol dm–3 NaOH(aq). What is the pH of the resulting solution? A 1.0 B 1.2 C 1.3 D 1.5 Your answer [1]
  23. June 2022, Paper 1, Q24 (1 mark): Ethanoic acid is a weak acid and hydrochloric acid is a strong acid. What is correct about 10 cm3 of 0.1 mol dm–3 solutions of each acid? A They both fizz at th
  24. June 2022, Paper 2, Q5(a)(i) (2 marks): Write the expression for Ka for HF and give its units. Ka = units [2]
  25. June 2022, Paper 2, Q5(a)(ii) (3 marks): Calculate the [F–] [HF] ratio in hydrofluoric acid under the acidic conditions in plaque at pH 5.0. [F–] [HF] = [3]
  26. June 2022, Paper 2, Q5(b) (2 marks): The HPO4 2– ion and explain your answer. Write the formula of the conjugate base of the HPO4 [2]
  27. November 2021, Paper 1, Q19 (1 mark): What is correct for 0.1 mol dm–3 ethanoic acid and 0.1 mol dm–3 hydrochloric acid? A Ethanoic acid and hydrochloric acid both dissociate fully in solution. B Hy
  28. November 2021, Paper 3, Q4(a)(i) (2 marks): Investigate, by calculation, whether the starting pH of 2.5 recorded for the propanoic acid solution is the expected value. [2]
  29. November 2021, Paper 3, Q4(a)(ii) (2 marks): A student says that the pH of propanoic acid with the added HCl (aq) is the same as if the 20 cm3 of 0.05 mol dm–3 HCl (aq) had been diluted with 30 cm3 water.
  30. November 2021, Paper 3, Q4(b) (3 marks): Explain, using an appropriate equation, why the sodium propanoate solution is alkaline. In your explanation you should identify the species behaving as a base a
  31. November 2021, Paper 3, Q4(c) (4 marks): Investigate, by calculation, whether the starting pH of 5.1 recorded for the solution of propanoic acid with dissolved sodium propanoate is the expected value.
  32. November 2021, Paper 3, Q4(d) (6 marks): This question refers to the Practical Insert that is provided as an insert to this paper. For propanoic acid, C2H5COOH, Ka = 1.3 × 10–5 mol dm–3. (d)* One of th
  33. November 2020, Paper 1, Q17 (1 mark): What is the pH of a 0.005 mol dm−3 solution of NaOH? A 1.3 B 2.3 C 11.7 D 12.7 Your answer [1]
  34. November 2020, Paper 1, Q32(d)(ii) (1 mark): Write an equation to show why hydrogen chloride dissolves in water to form an acidic solution. [1]
  35. November 2020, Paper 2, Q3(c)(i) (1 mark): Identify the base in this reaction and explain your choice. [1]
  36. November 2020, Paper 2, Q3(c)(ii) (1 mark): Calculate the pKa for the equilibrium in equation 3.1. pKa = [1] 10 Equation 3.1 is repeated again. − H+ + HPO4 2− Ka = 6.2 × 10−8 mol dm−3 Equation 3.1 H2PO4
  37. November 2020, Paper 2, Q3(c)(iii) (2 marks): Calculate the pH of a 1.0 × 10−3 mol dm−3 solution of KH2PO4. pH = [2]
  38. November 2020, Paper 2, Q3(e) (4 marks): A mixture of HPO4 Calculate the mass of KH2PO4 that must be added to 1.0 dm3 of a 0.10 mol dm−3 solution of 2− to make a solution with a pH of 6.50. HPO4 The vo
  39. November 2020, Paper 2, Q3(f)(i) (2 marks): A student says that a solution of HPO4 The student says that this is because the position of equilibrium in equation 3.1 moves to the left to remove H+ ions. Di
  40. November 2020, Paper 2, Q3(f)(ii) (1 mark): A student is doing calculations involving solutions of hydrochloric acid. The student says that the concentration of hydrogen ions can be taken as the concentra
  41. November 2020, Paper 2, Q3(f)(iii) (2 marks): Calculate the change in pH when 1 drop (0.05 cm3) of 0.01 mol dm−3 HCl is added to 1.0 dm3 of water at pH 7.0. pH change = [2]
  42. June 2019, Paper 1, Q18 (1 mark): A vinegar contains 5.0 g of ethanoic acid in each 100 cm3 of solution and no other acids. What is the pH of this vinegar? For ethanoic acid Ka = 1.7 × 10–5 mol
  43. June 2019, Paper 3, Q3(a) (2 marks): The solution in experiment B is behaving as a buffer solution. Explain the meaning of the term buffer solution. [2]
  44. June 2019, Paper 3, Q3(b) (2 marks): Ka for ethanoic acid is 1.7 × 10−5 mol dm−3. Show by calculation that the initial pH in experiment B is 4.8. [2]
  45. June 2019, Paper 3, Q3(c) (2 marks): Explain why the pH of sodium ethanoate in experiment C is alkaline. Include an equation in your answer. [2]
  46. June 2019, Paper 3, Q3(d) (3 marks): Calculate the pH of the solution formed after the addition of sodium hydroxide solution in experiment D. pH = [3]
  47. June 2019, Paper 3, Q3(e) (3 marks): In a follow-up experiment, 25.0 cm3 of the ethanoic acid solution is titrated with a solution of sodium hydroxide of unknown concentration and the following gra
  48. June 2018, Paper 1, Q26 (1 mark): Which statement about a strong base at 298 k is correct? A It is partially ionised. Its pH is given by pH = 14 + log[OH–]. B C It will not react with weak acids
  49. June 2018, Paper 2, Q4(a)(ii) (1 mark): HCO3 −. Give the formula of the conjugate base of HCO3 [1]
  50. June 2018, Paper 2, Q4(d)(i) (2 marks): A saturated solution of CO2 at 298 K has a concentration of 3.3 × 10−2 mol dm−3. Calculate the pH of this solution. pH = [2]
  51. June 2018, Paper 2, Q4(d)(ii) (1 mark): Calculate the concentration of a solution of HCl that has the same pH as the solution in (i). concentration = mol dm−3 [1]
  52. June 2018, Paper 2, Q4(e)(i) (2 marks): The pH of healthy human blood is 7.4. −] [HCO3 Calculate the ratio of [CO2] in healthy human blood. −] [HCO3 [CO2] = [2]
  53. June 2018, Paper 2, Q4(e)(ii) (2 marks): A patient’s blood has a pH below 7.4. A student says that HCO3 the patient’s blood. Say, with reasons, whether the student is correct. [2]
  54. June 2018, Paper 2, Q4(f) (3 marks): Some students mix 20 cm3 of 5.0 × 10−3 mol dm−3 HCl with 20 cm3 of 1.0 × 10−2 mol dm−3 NaOH. Calculate the pH of the resulting solution. pH = [3]
  55. June 2017, Paper 1, Q34(e) (2 marks): The indicator dye is a weak acid and may be described as HIn. For the reaction shown below, Ka = 1.6 × 10–4 mol dm–3. H+ + In– HIn This indicator changes colour
  56. June 2017, Paper 2, Q1(b) (1 mark): The ethanoate ion forms an alkaline solution in water. Write an equation to show this. [1]
  57. June 2017, Paper 2, Q1(c)(i) (1 mark): Write an equation for the reaction of ethanoic acid in water. [1]
  58. June 2017, Paper 2, Q1(c)(ii) (2 marks): Calculate the pH of a 0.030 mol dm–3 solution of ethanoic acid. pH = [2]
  59. June 2017, Paper 2, Q1(d) (3 marks): When sodium ethanoate is acting as an acidity regulator in food, a buffer solution is set up. This buffer involves sodium ethanoate and ethanoic acid. Explain,
  60. June 2017, Paper 2, Q1(e)(i) (1 mark): They make a solution containing equal amounts of ethanoic acid and sodium ethanoate. Calculate the pH of this solution. Ka = 1.7 × 10–5 mol dm–3 for ethanoic ac
  61. June 2017, Paper 2, Q1(e)(ii) (4 marks): The students then set out to make a buffer solution of a known pH. They have 25.0 cm3 of a 0.100 mol dm–3 solution of ethanoic acid. Calculate the mass of sodiu
  62. June 2017, Paper 2, Q5(c)(i) (1 mark): What is acting as the acid in the equation shown below? [Al(H2O)6]3+ + H2O → [Al(OH)(H2O)5]2+ + H3O+ [1]

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