plumiWeak acids and bases Ka for weak acids: AQA A‑Level Chemistry questions
AQA A-Level Chemistry past-paper questions on 3.1.12.4 Weak acids and bases Ka for weak acids, 2008–2025, with mark schemes and examiner’s reports where available.
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Q08.2
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All 76 questions on Weak acids and bases Ka for weak acids, as a list
- June 2025, Paper 1, Q08.2 (4 marks): Propanoic acid is a weak acid. Calculate the concentration of a solution of propanoic acid with pH = 2.89 at 25 °C The acid dissociation constant (Ka) for propa
- June 2024, Paper 3, Q04.1 (1 mark): Propanoic acid is a weak acid. State the meaning of weak in this context. [1 mark]
- June 2024, Paper 3, Q04.3 (3 marks): Give an expression for Ka for propanoic acid (CH3CH2COOH). Use this expression to show that pH = pKa when half of the propanoic acid has reacted with sodium hyd
- June 2024, Paper 3, Q04.4 (2 marks): Use the pH from Figure 4, when half of the propanoic acid has reacted, to calculate Ka at 298 K [2 marks] Ka mol dm–3
- June 2023, Paper 1, Q10.2 (1 mark): Give the expression for the acid dissociation constant (Ka) for propanoic acid (CH3CH2COOH). [1 mark] Ka
- June 2023, Paper 1, Q10.3 (4 marks): Calculate the pH of a 0.100 mol dm–3 propanoic acid solution. Give your answer to 2 decimal places. For propanoic acid, pKa = 4.87 [4 marks] pH
- June 2023, Paper 2, Q01.3 (3 marks): Chloroethanoic acid is a stronger acid than ethanoic acid. Sketch, on Figure 2, the curve you would expect when 25.0 cm3 of a 2.23 mol dm–3 solution of chloroet
- June 2023, Paper 3, Q13 (1 mark): When HF is added to water at 298 K, this equilibrium is established. HF(aq) ⇌ H+(aq) + F–(aq) At equilibrium, [HF] = 7.70 × 10–3 mol dm–3 and [F–] = 2.30 × 10–3
- June 2023, Paper 3, Q18 (1 mark): A 0.100 mol dm–3 solution of a weak acid has pH = 2.50 What is the value of Ka for this acid, in mol dm–3 ? [1 mark] A 3.16 × 10–2 B 3.16 × 10–3 C 1.00 × 10–4 D
- June 2022, Paper 1, Q04.1 (3 marks): Calculate the pH of a 0.150 mol dm–3 solution of ethanoic acid at 25 oC Give your answer to 2 decimal places. For ethanoic acid, Ka = 1.74 x 10–5 mol dm–3 at 25
- June 2022, Paper 3, Q15 (1 mark): Equal volumes of two solutions, each with the same concentration, are mixed together at 298 K Which two solutions, when mixed, form a solution with a pH >7? [1
- November 2021, Paper 3, Q05 (5 marks): A mixture of methanoic acid and sodium methanoate in aqueous solution acts as an acidic buffer solution. The equation shows the dissociation of methanoic acid.
- November 2020, Paper 1, Q04.1 (1 mark): State the meaning of the term weak acid. [1 mark]
- November 2020, Paper 1, Q04.2 (1 mark): Give an expression for the acid dissociation constant for propanoic acid. [1 mark] Ka
- November 2020, Paper 1, Q04.3 (4 marks): A student dilutes 25.0 cm3 of 0.500 mol dm–3 propanoic acid by adding water until the total volume is 100.0 cm3 Calculate the pH of this diluted solution of pro
- November 2020, Paper 3, Q14 (1 mark): A 0.10 mol dm–3 aqueous solution of an acid is added slowly to 25 cm3 of a 0.10 mol dm–3 aqueous solution of a base. Which acid–base pair has the highest pH at
- June 2019, Paper 1, Q09.2 (3 marks): Sodium hydroxide solution was added gradually from a burette to 25 cm3 of 0.080 mol dm–3 propanoic acid at 25 °C The pH was measured and recorded at regular int
- June 2019, Paper 3, Q02.2 (2 marks): In aqueous solution, sulfuric acid acts as a strong acid. The H2SO4 dissociates to form HSO4 − ions and H+ ions. The HSO4 − ions act as a weak acid and dissocia
- June 2019, Paper 3, Q02.4 (6 marks): A solution that contains 605 mg of NaHSO4 in 100 cm3 of solution has a pH of 1.72 Calculate the value of Ka for the hydrogensulfate ion (HSO4 −) that is behavin
- June 2019, Paper 3, Q02.5 (2 marks): Some sodium sulfate is dissolved in a sample of the solution from question 02.4. Explain why this increases the pH of the solution. [2 marks]
- June 2018, Paper 1, Q05.5 (1 mark): Give the expression for the acid dissociation constant (Ka) for ethanoic acid (CH3COOH). [1 mark] Ka
- June 2017, Paper 3, Q04.1 (1 mark): The pH range of an indicator is the range over which it changes colour. Suggest the pH range of a suitable indicator for this titration. [1 mark]
- June 2017, Paper 3, Q04.2 (1 mark): Give the expression for the acid dissociation constant of HX. [1 mark] Ka = 14
- June 2017, Paper 3, Q04.3 (2 marks): Calculate the concentration of HX in the original solution. [2 marks] Concentration mol dm−3
- June 2017, Paper 3, Q04.4 (2 marks): Calculate the pH of the solution of HX before the addition of any sodium hydroxide. (If you were unable to calculate a value for the concentration of HX in Ques
- June 2017, Paper 3, Q04.5 (1 mark): Calculate the pH of the solution when half of the acid has reacted. [1 mark] pH of solution
- June 2017, Paper 3, Q04.6 (2 marks): Plot your answers to Questions 4.4 and 4.5 on the grid in Figure 1. Use these points to sketch the missing part of the curve between 0 and 20 cm3 of NaOH soluti
- June 2017, Paper 3, Q09 (1 mark): 2,4,6-Trichlorophenol is a weak monoprotic acid, with Ka = 2.51 × 10−8 mol dm−3 at 298 K. What is the concentration, in mol dm–3, of hydrogen ions in a 2.00 × 1
- Specimen, Paper 1, Q06.1 (2 marks): Apparatus Explanation 5
- Specimen, Paper 1, Q06.2 (2 marks): Use information from the curve in Figure 1 to explain why the end point of this reaction would be difficult to judge accurately using an indicator. [2 marks] 0
- Specimen, Paper 1, Q06.4 (3 marks): Ka = mol dm
- Specimen, Paper 1, Q06.5 (2 marks): pH= 8
- Specimen, Paper 3, Q02.4 (1 mark): The pH ranges of three indicators are shown in Table 3. Table 3 Indicator pH range Bromocresol green 3.8–5.4 Bromothymol blue 6.0–7.6 Thymol blue 8.0–9.6 Select
- Specimen, Paper 3, Q04.1 (6 marks): Ethanedioic acid is a weak acid. Ethanedioic acid acts, initially, as a monoprotic acid. 4 – Use the concept of electronegativity to justify why the acid streng
- Specimen, Paper 3, Q14 (1 mark): The acid dissociation constant, Ka, of a weak acid HA has the value 2.56 × 10–4 mol dm–3 What is the pH of a 4.25 × 10–3 mol dm-3 solution of HA? [1 mark] A 5.9
- June 2016, CHEM4 (old spec), Q1(a) (1 mark): Write an equation to show how ethanoic acid behaves as a weak acid in its reaction with water. [1 mark]
- June 2016, CHEM4 (old spec), Q1(d)(i) (4 marks): Write an expression for Ka for ethanoic acid and use it to calculate the pH of the ethanoic acid solution in beaker C. Show your working. Give your answer to 2
- June 2016, CHEM4 (old spec), Q1(d)(iii) (3 marks): Calculate the value of –––––––––––– in the solution with the pH of 4.84 [3 marks] [salt] [ethanoic acid]
- June 2016, CHEM4 (old spec), Q1(e) (2 marks): Explain why chloroethanoic acid is a stronger acid than ethanoic acid. [2 marks]
- June 2016, CHEM4 (old spec), Q1(f) (2 marks): Explain why data books do not usually contain values of Ka for strong acids. [2 marks]
- June 2015, CHEM4 (old spec), Q3(a)(i) (1 mark): sodium hydroxide to 25 cm3 of ethanoic acid [1 mark] 3
- June 2015, CHEM4 (old spec), Q3(a)(ii) (1 mark): ammonia to 25 cm3 hydrobromic acid [1 mark] 3
- June 2015, CHEM4 (old spec), Q3(a)(iii) (1 mark): hydrochloric acid to 25 cm3 of potassium hydroxide [1 mark]
- June 2015, CHEM4 (old spec), Q3(b)(i) (1 mark): Which indicator in Table 2 could be used for the titration that produces curve E but not for the titration that produces curve F? [1 mark] Tick () one box. pen
- June 2015, CHEM4 (old spec), Q3(b)(ii) (1 mark): Give the colour change at the end point of the titration that produces curve H when naphthyl red is used as the indicator. [1 mark] 3
- June 2015, CHEM4 (old spec), Q3(b)(iii) (1 mark): A beaker contains 25 cm3 of a buffer solution at pH = 6.0 Two drops of each of the four indicators in Table 2 are added to this solution. State the colour of th
- June 2014, CHEM4 (old spec), Q2(a) (1 mark): Carboxylic acids are weak acids. State the meaning of the term weak as applied to carboxylic acids. [1 mark]
- June 2014, CHEM4 (old spec), Q2(d)(i) (1 mark): Write an expression for the acid dissociation constant Ka for benzenecarboxylic acid. [1 mark] 3
- June 2014, CHEM4 (old spec), Q2(d)(ii) (3 marks): Calculate the pH of a 0.0120 mol dm–3 solution of benzenecarboxylic acid. Give your answer to 2 decimal places. [3 marks]
- June 2014, CHEM4 (old spec), Q5(d) (2 marks): In practice, KCN rather than HCN is added to the carbonyl compound. Given that Ka for HCN = 4.0 × 10–10 mol dm–3, suggest why the reaction with HCN is very slow
- June 2013, CHEM4 (old spec), Q3(b)(i) (1 mark): Write an expression for the acid dissociation constant Ka for ethanoic acid. (1 mark) 5 3
- June 2013, CHEM4 (old spec), Q3(b)(ii) (4 marks): The value of Ka for ethanoic acid is 1.75 × 10–5 mol dm–3 at 25 ºC. Calculate the concentration of ethanoic acid in a solution of the acid that has a pH of 2.69
- June 2013, CHEM4 (old spec), Q3(c)(i) (1 mark): Write an equation for the dissociation of chloroethanoic acid in aqueous solution. (1 mark)
- June 2013, CHEM4 (old spec), Q3(c)(ii) (2 marks): Suggest why chloroethanoic acid is a stronger acid than ethanoic acid.
- January 2013, CHEM4 (old spec), Q2(b)(i) (1 mark): Write an equation for this dissociation. (1 mark)
- January 2013, CHEM4 (old spec), Q2(b)(ii) (1 mark): Write an expression for the acid dissociation constant Ka for methanoic acid. (1 mark)
- January 2013, CHEM4 (old spec), Q2(b)(iii) (3 marks): The value of Ka for methanoic acid is 1.78 × 10–4 mol dm–3 at 25 oC. Calculate the pH of a 0.0560 mol dm–3 solution of methanoic acid.
- January 2013, CHEM4 (old spec), Q2(b)(iv) (3 marks): The dissociation of methanoic acid in aqueous solution is endothermic. Deduce whether the pH of a solution of methanoic acid will increase, decrease or stay the
- June 2012, CHEM4 (old spec), Q3(d)(i) (1 mark): Calculate the value of pKa for HX at this temperature. Give your answer to 2 decimal places. (1 mark)
- June 2012, CHEM4 (old spec), Q3(d)(ii) (1 mark): Write an expression for the acid dissociation constant (Ka) for the weak acid HX. (1 mark) 3
- June 2012, CHEM4 (old spec), Q3(d)(iii) (3 marks): Calculate the pH of a 0.174 mol dm–3 solution of HX at this temperature. Give your answer to 2 decimal places.
- January 2012, CHEM4 (old spec), Q3(b)(ii) (1 mark): In terms of this reaction, state why the solution formed is weakly alkaline. (1 mark)
- January 2012, CHEM4 (old spec), Q4(b)(i) (1 mark): Write an expression for the acid dissociation constant, Ka, for HX. (1 mark) 3
- January 2012, CHEM4 (old spec), Q4(b)(ii) (3 marks): The pH of a 0.0850 mol dm–3 solution of HX is 2.79 Calculate a value for the acid dissociation constant, Ka, of this acid. Give your answer to 3 significant fig
- June 2011, CHEM4 (old spec), Q2(b)(i) (1 mark): Write an expression for Ka for ethanoic acid. (1 mark) 3
- June 2011, CHEM4 (old spec), Q2(b)(ii) (3 marks): Calculate the pH of a 0.154 mol dm–3 solution of ethanoic acid at 25 °C.
- June 2011, CHEM4 (old spec), Q2(c)(i) (4 marks): Calculate the pH of the solution formed when 10.0 cm3 of 0.154 mol dm–3 potassium hydroxide are added to 20.0 cm3 of 0.154 mol dm–3 ethanoic acid at 25 °C.
- January 2011, CHEM4 (old spec), Q2(b) (4 marks): The acid dissociation constant, Ka, for the weak acid HX has the value 4.83 x 10–5 mol dm–3 at 25 °C. A solution of HX has a pH of 2.48 Calculate the concentrat
- June 2010, CHEM4 (old spec), Q5(d)(i) (1 mark): Write an expression for the acid dissociation constant, Ka, for ethanoic acid. (1 mark) 5 3
- June 2010, CHEM4 (old spec), Q5(d)(ii) (3 marks): The value of Ka for ethanoic acid is 1.74 × 10–5moldm–3 at 25°C. Calculate the pH of a 0.136moldm–3 aqueous solution of ethanoic acid at this temperature.
- January 2010, CHEM4 (old spec), Q2(b)(iii) (1 mark): A list of indicators is shown below. Select from the list the most suitable indicator for the end-point of this titration. (1 mark) Indicator pH range thymol bl
- January 2010, CHEM4 (old spec), Q2(c)(i) (1 mark): Write an expression for the acid dissociation constant, Ka, for ethanoic acid. (1 mark) 3
- January 2010, CHEM4 (old spec), Q2(c)(ii) (3 marks): Calculate the pH of 0.410mol dm–3 ethanoic acid at this temperature.
- June 2008, CHM4 (2008 spec), Q1(c) (6 marks): 1 (c) HX is a weak Brønsted–Lowry acid. 1 (c) (i) Explain the terms weak and Brønsted–Lowry acid as applied to HX. Weak Brønsted–Lowry acid 1 (c) (ii) Write an
- June 2008, CHM5 (2008 spec), Q8(d) (4 marks): 8 (d) Aqueous solutions of AlCl3 are acidic. [Al(H2O)6]3+(aq) [Al(H2O)5(OH)]2+(aq) + H+(aq) The acid dissociation constant for this reaction, Ka = 1.26 × 10–5mo
- January 2008, CHM4 (2008 spec), Q2(a) (3 marks): (a) Calculate the pH of a 0.169 mol dm–3 solution of propanoic acid. (3 marks)
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