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Acid-base equilibria: WJEC A‑Level Chemistry questions

Every WJEC A-Level Chemistry exam question on 3.9 Acid-base equilibria, 2016–2025, with its mark scheme and examiner’s report.

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1,204 questions

  1. Q7(b)

    Summer 2025 · Unit 3

    2 marks3.9 Acid-base equilibria
    The whole paper ↗
    Summer 2025 Unit 3, question 7(b): Ammonium nitrate is a soluble salt. Suggest the pH of an aqueous solution of ammonium nitrate, giving a reason for your answer. [2]

  2. Q9(b)(i)

    Summer 2025 · Unit 3

    2 marks3.9 Acid-base equilibria
    The whole paper ↗
    Summer 2025 Unit 3, question 9(b)(i): Suggest what effect the presence of HI will have on the dissociation of CH3COOH in this mixture compared to an aqueous solution containing only CH3COOH. Give yo

  3. Q9(b)(iii)

    Summer 2025 · Unit 3

    6 marks3.9 Acid-base equilibria
    The whole paper ↗
    Summer 2025 Unit 3, question 9(b)(iii): 0.100 mol dm–3 and wishes to produce a buffer solution of pH 4.45. He has access to the full range of chemicals in a laboratory. Describe how the buffer solutio

  4. Q3

    Summer 2024 · Unit 3

    2 marks3.9 Acid-base equilibria
    The whole paper ↗
    Summer 2024 Unit 3, question 3: Calcium hydroxide, Ca(OH)2, is a strong base that is sparingly soluble in water. A saturated solution of calcium hydroxide has a concentration of 2.34 × 10–2 mo

  5. Q6(a)(i)

    Summer 2024 · Unit 3

    2 marks3.9 Acid-base equilibria
    The whole paper ↗
    Summer 2024 Unit 3, question 6(a)(i): State what is meant by a base and explain why the ammonia molecule is able to act as a base. [2]

  6. Q6(a)(ii)

    Summer 2024 · Unit 3

    2 marks3.9 Acid-base equilibria
    The whole paper ↗
    Summer 2024 Unit 3, question 6(a)(ii): Ammonia can be used as part of a mixture that forms a buffer. I. Suggest a compound that could be added to ammonia solution to form a buffer. [1] II. Suggest a

  7. Q10(c)(ii)

    Summer 2023 · Unit 3

    3 marks3.9 Acid-base equilibria
    The whole paper ↗
    Summer 2023 Unit 3, question 10(c)(ii): Calculate the pH of the HF solution formed. [3] Ka (HF) = 6.6 × 10−4 mol dm−3 pH =

  8. Q11(a)

    Summer 2023 · Unit 3

    1 mark3.9 Acid-base equilibria
    The whole paper ↗
    Summer 2023 Unit 3, question 11(a): State what is meant by the term weak acid. [1]

  9. Q11(b)

    Summer 2023 · Unit 3

    3 marks3.9 Acid-base equilibria
    The whole paper ↗
    Summer 2023 Unit 3, question 11(b): Substituted propanoic acids, CH3CHXCOOH, have different acid strengths. The table below gives some information about three substituted propanoic acids. Substitu

  10. Q11(c)(i)

    Summer 2023 · Unit 3

    2 marks3.9 Acid-base equilibria
    The whole paper ↗
    Summer 2023 Unit 3, question 11(c)(i): Calculate the initial pH of the aqueous sodium hydroxide. [2] pH =

  11. Q11(c)(ii)

    Summer 2023 · Unit 3

    2 marks3.9 Acid-base equilibria
    The whole paper ↗
    Summer 2023 Unit 3, question 11(c)(ii): Calculate the pH of the buffer solution. [2] pH =

  12. Q11(c)(iii)

    Summer 2023 · Unit 3

    3 marks3.9 Acid-base equilibria
    The whole paper ↗
    Summer 2023 Unit 3, question 11(c)(iii): Explain how buffers such as this work. [3]

  13. Q6

    Summer 2022 · Unit 3

    1 mark3.9 Acid-base equilibria
    The whole paper ↗
    Summer 2022 Unit 3, question 6: Write the expression for the ionic product of water, Kw. [1]

  14. Q10(a)

    Summer 2022 · Unit 3

    2 marks3.9 Acid-base equilibria
    The whole paper ↗
    Summer 2022 Unit 3, question 10(a): Several indicators are listed in the table. Identify all the indicator(s) which are appropriate for this titration. Give a reason for your choice(s). [2] Indica

  15. Q10(c)

    Summer 2022 · Unit 3

    4 marks3.9 Acid-base equilibria
    The whole paper ↗
    Summer 2022 Unit 3, question 10(c): hence calculate the initial pH of the HA solution. [4] You must show your working. pH after addition of 10.0 cm3 = Initial pH =

  16. Q10(d)

    Summer 2022 · Unit 3

    1 mark3.9 Acid-base equilibria
    The whole paper ↗
    Summer 2022 Unit 3, question 10(d): HA is a weak acid and so it can be used to make a buffer solution. Suggest one important use for buffer solutions. [1]

  17. Q10(e)

    Summer 2022 · Unit 3

    3 marks3.7 Entropy and feasibility of reactions3.9 Acid-base equilibria
    The whole paper ↗
    Summer 2022 Unit 3, question 10(e): Some data regarding the dissociation of another weak acid, HX, at two different temperatures are given in the table. H+ X– HX + HX HX at 298 K at temperature T

  18. Q11(c)(i)

    Summer 2022 · Unit 3

    1 mark3.9 Acid-base equilibria
    The whole paper ↗
    Summer 2022 Unit 3, question 11(c)(i): State how the Ka value of a stronger acid will compare with that of a weaker acid, giving a reason. [1]

  19. Q11(c)(iii)

    Summer 2022 · Unit 3

    2 marks3.9 Acid-base equilibria
    The whole paper ↗
    Summer 2022 Unit 3, question 11(c)(iii): One common salt of perchloric acid is ammonium perchlorate. Suggest a pH value for a solution of ammonium perchlorate. Give a reason for your answer. [2]

  20. Q3

    Summer 2019 · Unit 3

    1 mark3.9 Acid-base equilibria
    The whole paper ↗
    Summer 2019 Unit 3, question 3: Write the expression for the ionic product of water, Kw. [1]
All 31 questions on Acid-base equilibria, as a list
  1. Summer 2025, Unit 3, Q7(b) (2 marks): Ammonium nitrate is a soluble salt. Suggest the pH of an aqueous solution of ammonium nitrate, giving a reason for your answer. [2]
  2. Summer 2025, Unit 3, Q9(b)(i) (2 marks): Suggest what effect the presence of HI will have on the dissociation of CH3COOH in this mixture compared to an aqueous solution containing only CH3COOH. Give yo
  3. Summer 2025, Unit 3, Q9(b)(iii) (6 marks): 0.100 mol dm–3 and wishes to produce a buffer solution of pH 4.45. He has access to the full range of chemicals in a laboratory. Describe how the buffer solutio
  4. Summer 2024, Unit 3, Q3 (2 marks): Calcium hydroxide, Ca(OH)2, is a strong base that is sparingly soluble in water. A saturated solution of calcium hydroxide has a concentration of 2.34 × 10–2 mo
  5. Summer 2024, Unit 3, Q6(a)(i) (2 marks): State what is meant by a base and explain why the ammonia molecule is able to act as a base. [2]
  6. Summer 2024, Unit 3, Q6(a)(ii) (2 marks): Ammonia can be used as part of a mixture that forms a buffer. I. Suggest a compound that could be added to ammonia solution to form a buffer. [1] II. Suggest a
  7. Summer 2023, Unit 3, Q10(c)(ii) (3 marks): Calculate the pH of the HF solution formed. [3] Ka (HF) = 6.6 × 10−4 mol dm−3 pH =
  8. Summer 2023, Unit 3, Q11(a) (1 mark): State what is meant by the term weak acid. [1]
  9. Summer 2023, Unit 3, Q11(b) (3 marks): Substituted propanoic acids, CH3CHXCOOH, have different acid strengths. The table below gives some information about three substituted propanoic acids. Substitu
  10. Summer 2023, Unit 3, Q11(c)(i) (2 marks): Calculate the initial pH of the aqueous sodium hydroxide. [2] pH =
  11. Summer 2023, Unit 3, Q11(c)(ii) (2 marks): Calculate the pH of the buffer solution. [2] pH =
  12. Summer 2023, Unit 3, Q11(c)(iii) (3 marks): Explain how buffers such as this work. [3]
  13. Summer 2022, Unit 3, Q6 (1 mark): Write the expression for the ionic product of water, Kw. [1]
  14. Summer 2022, Unit 3, Q10(a) (2 marks): Several indicators are listed in the table. Identify all the indicator(s) which are appropriate for this titration. Give a reason for your choice(s). [2] Indica
  15. Summer 2022, Unit 3, Q10(c) (4 marks): hence calculate the initial pH of the HA solution. [4] You must show your working. pH after addition of 10.0 cm3 = Initial pH =
  16. Summer 2022, Unit 3, Q10(d) (1 mark): HA is a weak acid and so it can be used to make a buffer solution. Suggest one important use for buffer solutions. [1]
  17. Summer 2022, Unit 3, Q10(e) (3 marks): Some data regarding the dissociation of another weak acid, HX, at two different temperatures are given in the table. H+ X– HX + HX HX at 298 K at temperature T
  18. Summer 2022, Unit 3, Q11(c)(i) (1 mark): State how the Ka value of a stronger acid will compare with that of a weaker acid, giving a reason. [1]
  19. Summer 2022, Unit 3, Q11(c)(iii) (2 marks): One common salt of perchloric acid is ammonium perchlorate. Suggest a pH value for a solution of ammonium perchlorate. Give a reason for your answer. [2]
  20. Summer 2019, Unit 3, Q3 (1 mark): Write the expression for the ionic product of water, Kw. [1]
  21. Summer 2019, Unit 3, Q10(c)(ii) (3 marks): aqueous solution of ethanoic acid with a concentration of 0.220 mol dm–3. [3] pH =
  22. Summer 2019, Unit 3, Q11(b) (6 marks): Oxalic acid solutions may be analysed using acid-base titrations. The simplified pH curve for the titration of a solution of oxalic acid against sodium hydroxid
  23. Summer 2018, Unit 3, Q7 (2 marks): When a solution containing the weak base ammonia is neutralised using the strong acid sulfuric acid, a solution of ammonium sulfate is formed. Suggest a pH for
  24. Summer 2018, Unit 3, Q8(b)(i) (1 mark): Write an expression for the Ka of hydrofluoric acid. [1]
  25. Summer 2018, Unit 3, Q8(b)(ii) (3 marks): [3] pH =
  26. Summer 2018, Unit 3, Q8(b)(iii) (2 marks): The concentration of a solution of hydrofluoric acid can be found by titrating against sodium hydroxide. Not all acid-base indicators would be suitable for this
  27. Summer 2018, Unit 3, Q8(b)(iv) (5 marks): acid of concentration 0.100 mol dm–3 forms a buffer solution. I. Explain how this buffer solution works. [3] II. Calculate the pH of this buffer solution. [2] p
  28. Summer 2017, Unit 3, Q7(a) (1 mark): Ammonia is an example of a weak base. Describe what is meant by the term weak base. [1]
  29. Summer 2017, Unit 3, Q7(b) (3 marks): A mixture of ammonia and ammonium chloride in aqueous solution can be used as a basic buffer solution. Explain what is meant by a buffer solution and how this m
  30. Summer 2017, Unit 3, Q7(c)(i) (2 marks): Calculate Ka for the ammonium ion. [2] Ka = mol dm−3
  31. Summer 2017, Unit 3, Q7(c)(ii) (1 mark): Select an appropriate indicator for this titration from the list below, giving a reason for your answer. [1] Indicator pH range bromothymol blue 6.0 – 7.6 4-nit

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