plumiAcid-base equilibria: Edexcel A‑Level Chemistry questions
Every Edexcel A-Level Chemistry exam question on 12 Acid-base equilibria, 2016–2025, with its mark scheme and examiner’s report.
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Q5(a)
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Q5(b)
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Q5(c)
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Q8(a)
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Q8(b)(i)
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Q8(b)(ii)
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Q8(b)(iii)
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Q8(b)(iv)
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Q8(c)
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Q8(d)(i)
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Q8(d)(ii)
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Q4(a)
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Q4(b)
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Q4(c)
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Q4(d)(i)
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Q4(d)(ii)
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Q4(e)(i)
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Q4(e)(ii)
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Q8(a)(i)
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Q8(a)(ii)
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All 85 questions on Acid-base equilibria, as a list
- June 2025, Paper 1, Q5(a) (1 mark): The reaction of nitric acid with water can be represented by the equation shown. HNO3(aq) + H2O(l) H3O+(aq) + NO3 –(aq) Which is a conjugate acid–base pair fo
- June 2025, Paper 1, Q5(b) (1 mark): State the expression that defines the pH of a solution. (1)
- June 2025, Paper 1, Q5(c) (1 mark): What is the concentration of OH–(aq) ions, in mol dm–3, in a solution of Ba(OH)2(aq) with pH = 9.50 at 298 K? (1) A 3.16 × 10–10 B 3.16 × 10–5 C 6.32 × 10–5 D 3
- June 2025, Paper 3, Q8(a) (2 marks): An aqueous solution of a weak acid, HA(aq), with a concentration of 0.0712 mol dm−3, has a pH = 3.25 at room temperature. Calculate the value of Ka for the weak
- June 2025, Paper 3, Q8(b)(i) (1 mark): Calculate the pH of the sodium hydroxide solution. Give your answer to one decimal place. [Kw = 1.00 × 10−14 mol2 dm−6] (1)
- June 2025, Paper 3, Q8(b)(ii) (3 marks): Draw a titration curve, on the axes shown, to show the change in pH throughout the experiment. (3) 14 12 10 8 pH 6 4 2 0 0 10 20 30 40 Volume of NaOH(aq) / cm3
- June 2025, Paper 3, Q8(b)(iii) (1 mark): Give an estimate of the pH value at the half-equivalence point. Show your working on the titration curve. (1)
- June 2025, Paper 3, Q8(b)(iv) (3 marks): Calculate a value for Ka of ethanoic acid using your answer to (b)(iii). You must show your working. (3)
- June 2025, Paper 3, Q8(c) (3 marks): Another method used to monitor the titration is to use a pH meter without a data logger. The pH is measured and recorded after the addition of each 1.00 cm3 por
- June 2025, Paper 3, Q8(d)(i) (4 marks): 2.30 g of sodium propanoate is dissolved in propanoic acid solution. This forms a buffer with pH = 4.2 and a volume of exactly 100 cm3. Calculate the concentrat
- June 2025, Paper 3, Q8(d)(ii) (3 marks): Explain why there is no significant change in pH when a small amount of hydrochloric acid, HCl(aq), is added to this buffer. (3)
- June 2024, Paper 1, Q4(a) (1 mark): State what is meant by a Brønsted−Lowry acid. (1)
- June 2024, Paper 1, Q4(b) (1 mark): Which could be the titration curve when 0.100 mol dm−3 NaOH(aq) is added to 25.0 cm3 of 0.100 mol dm−3 CH3COOH(aq)? (1) A B 14 14 12 12 10 10 8 8 pH pH 6 6 4 4
- June 2024, Paper 1, Q4(c) (4 marks): Some information about acids in aqueous solution is given. Comment on these pH values. No calculations are required. (4) pH of a solution of Name of acid Formul
- June 2024, Paper 1, Q4(d)(i) (1 mark): Write the expression for the acid dissociation constant, Ka, for carbonic acid. State symbols are not required. (1)
- June 2024, Paper 1, Q4(d)(ii) (3 marks): A blood sample taken from an individual was analysed. Calculate the pH of the blood sample. Use your expression for Ka and the values shown. Ka for carbonic aci
- June 2024, Paper 1, Q4(e)(i) (2 marks): When a person exercises vigorously the concentration of carbon dioxide (aq) in the blood increases. Explain how this increase in the concentration of carbon dio
- June 2024, Paper 1, Q4(e)(ii) (2 marks): Explain how the carbonic acid–hydrogencarbonate buffer system in Equilibrium 2 acts to restore the pH of the blood after a person has exercised. (2)
- June 2024, Paper 3, Q8(a)(i) (3 marks): Explain why this procedure would give invalid results. (3)
- June 2024, Paper 3, Q8(a)(ii) (1 mark): State how Step 1 should be amended to give valid results. (1)
- June 2024, Paper 3, Q8(a)(iii) (3 marks): Show that when the solution has been half-neutralised, the acid dissociation constant is given by the expression Ka = 10–pH (3)
- June 2024, Paper 3, Q8(b) (5 marks): Calculate the mass of sodium ethanoate needed to be dissolved in 250 cm3 of 0.520 mol dm–3 ethanoic acid to form a buffer solution with pH = 4.48 [Ka for ethano
- June 2024, Paper 3, Q8(c) (5 marks): A student mixes 50.0 cm3 of 0.900 mol dm–3 NaOH(aq) with 20.0 cm3 of 0.400 mol dm–3 H2SO4(aq). Calculate the pH of the resultant solution. [Ionic product of wat
- June 2023, Paper 1, Q6(a) (1 mark): Which of these answers identifies the types of species present when benzoic acid is mixed with nitric acid? [Ka of benzoic acid = 6.3 × 10–5 mol dm–3; Ka of nit
- June 2023, Paper 1, Q6(c)(i) (1 mark): Write the expression for the acid dissociation constant, Ka , of benzoic acid. (1)
- June 2023, Paper 1, Q6(c)(ii) (4 marks): Calculate the mass of benzoic acid needed to prepare 250 cm3 of a solution with a pH = 3.51 (4)
- June 2023, Paper 1, Q6(d)(i) (1 mark): Which of these could be used to show the end‑point of a titration of benzoic acid with sodium hydroxide solution? (1) A bromothymol blue B litmus C methyl orang
- June 2023, Paper 1, Q6(d)(iii) (2 marks): The standard enthalpy change of neutralisation of the weak acid HCN by sodium hydroxide is –11.7 kJ mol–1 while that of the strong acid HCl is –57.9 kJ mol–1. E
- June 2023, Paper 3, Q7(b)(ii) (4 marks): The pH 10 buffer can be made by adding solid ammonium chloride to an aqueous solution of ammonia of concentration 18.1 mol dm–3. The relevant equation is NH4 +
- June 2023, Paper 3, Q7(b)(iii) (1 mark): State a necessary laboratory precaution, other than wearing a laboratory coat, gloves and goggles, that must be taken when using concentrated ammonia. (1)
- June 2022, Paper 1, Q6(a) (1 mark): State what is meant by a Brønsted‑Lowry base. (1)
- June 2022, Paper 1, Q6(c) (1 mark): Calculate the concentration of hydrogen ions, in mol dm pH of 9.43 (1)
- June 2022, Paper 1, Q6(d) (1 mark): The pH of two salt solutions, J and K, are solution J pH = 5 solution K pH = 9 The solutions are equimolar. Which acids and bases could form the salts in soluti
- June 2022, Paper 1, Q6(e)(i) (3 marks): Determine the value of Kw at 45 °C by plotting a suitable graph. You must show your working on the graph. (3) Kw at 45 °C =
- June 2022, Paper 1, Q6(e)(ii) (3 marks): The ionic product of water at 30 °C is 1.47 × 10–14 mol2 dm–6. Calculate the pH of water at this temperature. (3)
- June 2022, Paper 1, Q6(f)(i) (2 marks): Deduce a suitable indicator to identify the first equivalence point. Justify your answer using values from the Data Booklet. (2)
- June 2022, Paper 1, Q6(f)(ii) (1 mark): Write the equation for the reaction taking place at the second equivalence point. State symbols are not required. (1)
- June 2022, Paper 1, Q6(f)(iii) (3 marks): Explain how the solution at point X on the graph can act as a buffer solution. (3)
- June 2022, Paper 3, Q8(a) (5 marks): Devise an experiment to determine the acid dissociation constant, Ka , for a solution of ethanoic acid, CH3COOH, of unknown concentration. Assume you have acces
- June 2022, Paper 3, Q8(b) (3 marks): 500 cm3 of a buffer solution of pH = 4.70 is required. Calculate the volume of 0.800 mol dm–3 sodium ethanoate solution and of 0.800 mol dm–3 ethanoic acid need
- June 2022, Paper 3, Q8(c) (6 marks): Calculate the pH of the solution formed when 51.2 cm3 of 0.927 mol dm–3 NaOH(aq) is mixed with 40.4 cm3 of 0.370 mol dm–3 H2SO4(aq). [Ionic product of water Kw
- June 2022, Paper 3, Q9(a)(i) (1 mark): Give a reason why methyl orange would not be a suitable indicator to use in this titration. (1)
- November 2021, Paper 1, Q7(a) (1 mark): Ethanoic acid, CH3COOH, is a monobasic acid. CH3COOH + H2O CH3COO– + H3O+ Give a reason why only the proton from the carboxylic acid group, and not from the m
- November 2021, Paper 1, Q7(b) (1 mark): The reaction of ammonia with water can be represented by NH3 + H2O NH4 + + OH– Which is the acid‑conjugate base pair? (1) Acid Conjugate base A OH– NH3 B + NH
- November 2021, Paper 1, Q7(c) (2 marks): A commercial nitric acid solution, HNO3(aq), has a concentration of 15.9 mol dm–3. A 15.0 cm3 sample was made up to 100 cm3 by adding deionised water. Calculate
- November 2021, Paper 1, Q7(d)(i) (3 marks): Calculate the pH of a 0.100 mol dm–3 solution of propanoic acid at 298 K. Give your answer to an appropriate number of significant figures. [Ka = 1.35 × 10–5 mo
- November 2021, Paper 1, Q7(d)(ii) (2 marks): State two assumptions that you made in the calculation in d(i). (2)
- November 2021, Paper 1, Q7(e)(i) (4 marks): Calculate the pH of this buffer solution at 298 K. (4)
- November 2021, Paper 1, Q7(e)(ii) (2 marks): Explain why the pH of the buffer solution hardly changes when a few drops of sodium hydroxide solution are added to it. Include an equation or equations in your
- October 2020, Paper 1, Q2(a) (1 mark): What is the order of decreasing pH for 0.100 mol dm−3 solutions of these three acids? (1) A CH3COOH > CH2ClCOOH > HCl B HCl > CH3COOH > CH2ClCOOH C CH2ClCOOH >
- October 2020, Paper 1, Q2(b) (3 marks): A solution of methanoic acid, HCOOH, has a concentration of 0.240 mol dm−3 and a pH of 2.20. Calculate the value of pKa for methanoic acid. (3)
- October 2020, Paper 1, Q2(c) (1 mark): Which of these mixtures would form a buffer solution with a pH below 7? (1) A NaOH(aq) and excess HCl(aq) B NaOH(aq) and excess CH3COOH(aq) C excess NaOH(aq) an
- October 2020, Paper 1, Q2(d) (1 mark): Bromothymol blue, methyl orange and phenolphthalein are indicators used in titrations. Which, if any, of these indicators could be used for a titration of ammon
- October 2020, Paper 3, Q3(c)(i) (4 marks): Assess the suitability of methyl red as an indicator for this titration. Make use of the Data Booklet in answering this question. (4)
- October 2020, Paper 3, Q3(c)(ii) (1 mark): Complete the table, with a tick (✓) or a cross (✗), to show whether or not the indicator would be suitable for use in this titration. (1) Indicator pH range Tic
- June 2019, Paper 1, Q3(b)(ii) (2 marks): Explain why the data book value for the standard enthalpy change of neutralisation of ethanoic acid with sodium hydroxide is –55.2 kJ mol–1 but the value for hy
- June 2019, Paper 1, Q6(d)(i) (2 marks): Calculate the pH of water at 310 K. Give your answer to two decimal places. (2)
- June 2019, Paper 1, Q6(d)(ii) (2 marks): Predict, with a reason, whether water is acidic, alkaline or neutral at 310 K. (2)
- June 2019, Paper 1, Q6(d)(iii) (1 mark): Predict, with a reason, the sign of the enthalpy change for the ionisation of water. (1)
- June 2019, Paper 1, Q9(a) (1 mark): A buffer solution always (1) A keeps the pH less than 7. B contains equimolar amounts of acid and its conjugate base. C keeps the pH constant if small quantitie
- June 2019, Paper 1, Q9(b) (5 marks): A buffer solution with a pH of 3.90 is required. Calculate the mass, in grams, of sodium ethanoate that should be added to 50.0 cm3 of an ethanoic acid solution
- June 2019, Paper 1, Q9(c) (3 marks): One of the systems controlling the pH of blood is the carbonic acid-hydrogencarbonate buffer system. ` H+ + HCO3 – H2CO3 _ Explain how this buffer system helps
- June 2019, Paper 1, Q9(d)(i) (4 marks): Draw a titration curve for the addition of 100 cm3 of sodium hydroxide solution of concentration 0.100 mol dm–3 to 40.0 cm3 of propanoic acid solution of concen
- June 2019, Paper 1, Q9(d)(ii) (2 marks): Describe, without calculation, how you would use your curve to determine the value of Ka for propanoic acid. (2)
- June 2019, Paper 3, Q1(a) (1 mark): State what is meant by a Brønsted-Lowry acid. (1)
- June 2019, Paper 3, Q1(b) (1 mark): Identify the acid-base conjugate pairs in this reaction. (1) CH3COOH + HCOOH o CH3COOH2 + + HCOO–
- June 2019, Paper 3, Q1(c) (1 mark): Write the expression that defines the pH of a solution. (1)
- June 2019, Paper 3, Q1(d) (1 mark): Calculate the concentration of hydrogen ions, in mol dm–3, in a solution with a pH of 2.76 (1)
- June 2019, Paper 3, Q1(e) (2 marks): Explain why the pH of a 1 × 10–8 mol dm–3 solution of nitric acid, HNO3 , is not 8. [Ionic product of water, Kw = 1.00 × 10–14 mol2 dm–6] (2)
- June 2018, Paper 1, Q7(d)(i) (3 marks): Calculate the pH of a 0.0500 mol dm−3 solution of boric acid from the pKa value for the first dissociation. (3)
- June 2018, Paper 1, Q7(d)(ii) (2 marks): State any assumptions you made in your calculation in (d)(i). (2)
- June 2018, Paper 1, Q7(e) (1 mark): Boric acid can undergo further dissociation. Which is the conjugate acid of the HBO3 2− ion? (1) A BO3 3− B H2BO3 − C H3BO3 D H3O+
- June 2018, Paper 3, Q7(a) (5 marks): A weak acid, HX, has a Ka value of 5.25 × 10−5 mol dm−3. A solution was formed by mixing 10.5 cm3 of 0.800 mol dm−3 dilute sodium hydroxide with 25.0 cm3 of 0.9
- June 2018, Paper 3, Q7(b)(i) (4 marks): Propanoic acid, CH3CH2COOH, is a weak acid. On the grid below, sketch the change in pH during the addition of 50.0 cm3 of 0.100 mol dm−3 sodium hydroxide soluti
- June 2018, Paper 3, Q7(b)(ii) (2 marks): Explain how you would use the graph in (b)(i) to obtain the value of the acid dissociation constant, Ka , for propanoic acid. You are not expected to calculate
- June 2017, Paper 1, Q7(b) (6 marks): The table shows the enthalpy changes of reaction when 1 mol of different acids are neutralised by sodium hydroxide solution, at 298 K. Enthalpy change of reacti
- June 2017, Paper 1, Q8(a) (1 mark): A solution of glycolic acid of concentration 0.1 mol dm–3 has a pH of 2.4 What is the approximate pH of the resulting solution after it has been diluted by a fa
- June 2017, Paper 1, Q8(b) (3 marks): Another solution of glycolic acid has a pH of 2.0 Calculate the concentration of this solution. (3)
- June 2017, Paper 1, Q8(c)(i) (3 marks): Use the information given in your Data Booklet to select a suitable indicator for this titration, including the colour change you would expect to see. Justify y
- June 2017, Paper 1, Q8(c)(ii) (1 mark): What is the concentration of this glycolic acid in mol dm–3? (1) A 0.080 B 0.100 C 0.125 D 0.250
- June 2017, Paper 1, Q8(c)(iii) (1 mark): The pH of the solution containing just sodium glycolate and water is (1) A 2.8 B 6.0 C 8.3 D 11.0
- June 2017, Paper 1, Q8(d)(i) (2 marks): Give a possible explanation as to why the value of Ka for glycolic acid is approximately ten times larger than that of ethanoic acid. (2)
- June 2017, Paper 1, Q8(d)(ii) (1 mark): Complete the equation to show the conjugate acid-base pairs that would be produced when pure samples of glycolic acid and ethanoic acid are mixed. (1) CH2OHCOOH
- June 2017, Paper 1, Q9(a) (2 marks): A buffer solution is formed from disodium hydrogenphosphate, containing HPO4 – ions. and sodium dihydrogenphosphate, containing H2PO4 2– and H2PO4 – ions to sho
- June 2017, Paper 1, Q9(b) (5 marks): Another buffer solution was formed by mixing 20.0 cm3 of sodium hydroxide solution of concentration 0.100 mol dm–3 with 25.0 cm3 of ethanoic acid of concentrati
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