Acids, bases and pH: Edexcel International A Level Chemistry questions

Every Edexcel International A Level Chemistry exam question on Acids, bases and pH (specification points 14.1–14.14), 2019–2025, with its mark scheme and examiner’s report.

All 80 questions on Acids, bases and pH, as a list

June 2025, Unit 4, Q4 (1 mark): What is the pH of a 1.25 mol dm–3 solution of hydrochloric acid? A −1.25 B −0.097 C 0.097 D 1.25
June 2025, Unit 4, Q5 (1 mark): At 25 °C, the pH of pure water is 7.0 and at 75°C the pH is 6.4 . Why is the pH of pure water lower at 75 °C than at 25 °C? A there are more hydrogen ions than
June 2025, Unit 4, Q6 (1 mark): A sodium hydroxide solution with pH = 12 is diluted by a factor of 100. What is the pH of the diluted solution? A 10 B 11 C 13 D 14 Use this space for any rough
June 2025, Unit 4, Q17(a)(i) (1 mark): Write the expression for Ka for ethanoic acid, CH3COOH. (1)
June 2025, Unit 4, Q17(a)(ii) (2 marks): Calculate the pH of a 0.25 mol dm−3 solution of ethanoic acid. (2)
June 2025, Unit 4, Q17(a)(iii) (1 mark): State one assumption you have made when calculating the pH in (a)(ii). (1)
June 2025, Unit 4, Q17(b)(i) (1 mark): Suggest a reason why chloroethanoic acid, CH2ClCOOH, is a stronger acid than ethanoic acid. (1)
June 2025, Unit 4, Q17(b)(ii) (2 marks): When ethanoic acid and chloroethanoic acid are added together an equilibrium is set up containing two acid-base pairs. Complete the equilibrium equation, labell
January 2025, Unit 4, Q3 (1 mark): When ammonia dissolves in water, an equilibrium results. The equation for this equilibrium is shown. NH3 + H2O  NH+ 4 + OH– Which of these species act as a Brø
January 2025, Unit 4, Q4 (1 mark): A strong monoprotic acid, of concentration 0.0100 mol dm–3, has a pH = 2.0. What is the decrease in pH if the concentration is increased to 0.0110 mol dm–3? A 0
January 2025, Unit 4, Q11 (1 mark): What is the pH of a 0.100 mol dm–3 solution of sodium hydroxide at 283 K? [pKw = 14.53 at 283 K] A 13.00 B 13.53 C 14.43 D 14.53
January 2025, Unit 4, Q12 (1 mark): The pH of a dilute solution of a weak carboxylic acid can be calculated from its Ka value. Which is not a relevant assumption in the calculation of pH? A the io
October 2024, Unit 4, Q5 (1 mark): – . 5 Ethanoic acid, CH3COOH, can react with a hydrogensulfite ion, HSO3 Which is a correct acid conjugate‑base pair for this reaction? [Ka for ethanoic acid =
October 2024, Unit 4, Q8 (1 mark): The dissociation of water is an endothermic reaction. H2O(l)  H+(aq) + OH–(aq) ΔH is positive [Kw = 1.0 × 10–14 mol2 dm–6 at 298 K Kw = 2.92 × 10–14 mol2 dm–6
October 2024, Unit 4, Q17(a) (6 marks): Sulfuric acid reacts with sodium hydroxide solution as shown. H2SO4(aq) + 2NaOH(aq) → Na2SO4(aq) + 2H2O(l) Calculate the pH of the solution formed when 20.0 cm3
October 2024, Unit 4, Q17(b)(i) (1 mark): State what is meant by the term weak in this context. (1)
October 2024, Unit 6, Q3(b) (3 marks): Ethanedioic acid dissociates in two steps. The first dissociation equation is shown. (COOH)2(aq)  HCOOCOO–(aq) + H+(aq) The acid dissociation constant, Ka1 , f
June 2024, Unit 4, Q19(a)(i) (2 marks): Complete the table. (2)
June 2024, Unit 4, Q19(a)(ii) (1 mark): The structure of C5H5NH+ is shown. N+ H Write the Ka expression for this weak acid. (1)
June 2024, Unit 4, Q19(a)(iii) (2 marks): When methanoic acid is added to propanoic acid, an equilibrium is set up containing two acid‑base pairs. Complete the equilibrium, labelling the acid‑base pairs
June 2024, Unit 4, Q19(a)(iv) (6 marks): A student measured the pH of separate 0.050 mol dm–3 solutions of CH2ClCOOH and of CHCl2COOH, using a pH meter. The student also calculated the pH of these solu
June 2024, Unit 4, Q19(c) (5 marks): A student was given 50.0 cm3 of a solution of sodium hydroxide. The pH of this solution was 12.43. The student was asked to adjust the pH to 12.00, by dilution
January 2024, Unit 4, Q7 (1 mark): At 50 °C, the ionic product of water, Kw, is 5.5 × 10–14 mol2 dm–6. At this temperature, water is A neutral with a pH of 7.0 B neutral with a pH of 6.6 C acidic
January 2024, Unit 4, Q8 (1 mark): Equimolar solutions of four acids are prepared. Which solution has the lowest pH? Use electronegativity values from the Data Booklet. A CH3COOH B CH2ClCOOH C CH
January 2024, Unit 4, Q9 (1 mark): Some equations for acid‑base equilibria are shown. H3PO4 + H2O  H2PO4 – + H3O+ – + H2O  HPO4 2– + H3O+ H2PO4 HPO4 2– + H2O  PO4 3– + H3O+ 2–? What is the con
January 2024, Unit 4, Q10 (1 mark): What is the pH of a 0.200 mol dm–3 solution of strontium hydroxide, Sr(OH)2? Kw = 1.00 × 10–14 mol2 dm–6 A 14.0 B 13.6 C 13.3 D 12.6
January 2024, Unit 4, Q19(b)(i) (1 mark): State the difference between a weak acid and a strong acid such as hydrochloric acid. (1)
January 2024, Unit 4, Q19(b)(ii) (1 mark): Calculate the pH of 0.500 mol dm–3 hydrochloric acid at 25 °C. (1)
January 2024, Unit 4, Q19(b)(iii) (3 marks): Calculate the pH of 0.500 mol dm–3 propanoic acid at 25 °C. Ka (propanoic acid) = 1.30 × 10–5 mol dm–3 at 25 °C. (3)
October 2023, Unit 4, Q7 (1 mark): When concentrated hydrochloric acid is added to methanoic acid, an acid‑base reaction occurs. HCl + HCOOH  Cl– + HCOOH2 + What are the Brønsted‑Lowry acid‑base
October 2023, Unit 4, Q8 (1 mark): At 100 °C, pure water has a pH of 6, but at 25 °C it has a pH of 7. This is because A the dissociation of water is endothermic, so the concentration of hydrogen
October 2023, Unit 4, Q9 (1 mark): Four beakers each contain equal volumes of four solutions. Beaker 1 contains 0.01 mol dm–3 NaOH Beaker 2 contains 0.01 mol dm–3 NH3 Beaker 3 contains 0.1 mol dm
October 2023, Unit 4, Q18(a)(i) (3 marks): At Key point 1, before any potassium hydroxide has been added, the pH = 2.9. Use a calculation to justify this value. (3)
October 2023, Unit 4, Q18(a)(ii) (2 marks): The student deduced that the pH at Key point 4 must be less than 13. Use a calculation to justify the student’s deduction. You are not required to calculate the
October 2023, Unit 5, Q21(c)(i) (1 mark): Complete the equation to show the dissociation of acetylsalicylic acid in aqueous solution. (1) COOH ⇌ OCOCH3
October 2023, Unit 5, Q21(c)(ii) (3 marks): The dissociation of acetylsalicylic acid is different in the acidic conditions found in the stomach compared with the slightly alkaline conditions in the small
June 2023, Unit 4, Q15 (1 mark): Which is not a conjugate acid-base pair? A NH3 , NH2 – B NH4 + , NH3 C H2CO3 , CO3 2– D H2CO3 , HCO3 –
June 2023, Unit 4, Q16 (1 mark): What is the pH of the solution when 2.15 g of barium hydroxide is dissolved in 200 cm3 of deionised water? [molar mass of barium hydroxide = 171.3 g mol–1 Kw =
June 2023, Unit 4, Q22(b)(ii) (1 mark): Write the expression for the Ka of the conjugate acid of Tris (C4H12NO3 (1)
June 2023, Unit 4, Q22(c) (4 marks): A solution of chloroethanoic acid is prepared for titration with Tris. 0.0150 g of chloroethanoic acid (Mr = 94.5) is dissolved in 1500 cm3 of distilled water.
January 2023, Unit 4, Q10 (1 mark): Equal amounts of ethanoic acid and methanoic acid are mixed. [pKa values: ethanoic acid = 4.8 methanoic acid = 3.8] What are the conjugate acid‑base pairs in th
January 2023, Unit 4, Q11 (1 mark): A 1 mol dm–3 solution of ethanoic acid is gradually diluted by the addition of distilled water. What happens to the degree of dissociation of the acid and the p
January 2023, Unit 4, Q12(a) (1 mark): What is the pH of a saturated solution of magnesium hydroxide at 25 °C? (1) A 9.7 B 10.0 C 10.3 D 12.0
January 2023, Unit 4, Q12(b) (1 mark): Saturated solutions of magnesium hydroxide at 25 °C and 100 °C are compared. How do the concentrations of hydroxide ions and the pH values of these solutions di
January 2023, Unit 4, Q21(a)(i) (1 mark): Write the equation for the acid dissociation constant, Ka, of gluconic acid. Use RCOOH for gluconic acid. (1)
January 2023, Unit 4, Q21(a)(ii) (4 marks): Calculate the pH of the solution of gluconic acid. [Mr (gluconic acid) = 196 Ka (gluconic acid) = 1.38 × 10–4 mol dm–3] (4)
October 2022, Unit 4, Q7 (1 mark): In which equation is H2O acting only as a Brønsted–Lowry acid? A CH3COOH + H2O → CH3COO− + H3O+ + + H2O → H3O+ + NH3 B NH4 C NH3 + H2O → NH4 + + OH− D 2H2O → H3
October 2022, Unit 4, Q8 (1 mark): What is the pH of a 0.050 mol dm−3 solution of barium hydroxide, Ba(OH)2 ? A 13.3 B 13.0 C 12.7 D 12.4
October 2022, Unit 4, Q15(d)(i) (4 marks): Calculate the mass, in g, of lactic acid, HA , required to make 1.00 dm3 of an aqueous solution with a pH of 3.00 . Assume: • [HA]initial = [HA]equilibrium [H+]
October 2022, Unit 4, Q15(d)(ii) (2 marks): In practice, the initial concentration of lactic acid does not equal its equilibrium concentration. Explain how this affects the mass of acid needed to make a s
October 2022, Unit 4, Q15(d)(iii) (1 mark): Lactic acid is a stronger acid than ethanoic acid. H H H O O O H C C H C C C O H O H H H H lactic acid ethanoic acid Give a possible reason why lactic acid is t
June 2022, Unit 4, Q7 (1 mark): What is the pH of a 0.010 mol dm−3 aqueous solution of carbonic acid? [Ka of carbonic acid = 4.5 × 10−7 mol dm−3 at 298 K] A 2.00 B 4.17 C 6.35 D 8.35
June 2022, Unit 4, Q8 (1 mark): What is the pH of a 0.27 mol dm−3 aqueous solution of sodium hydroxide? [Kw = 1.0 × 10−14 mol2 dm−6 at 298 K] A 0.57 B 13.43 C 13.73 D 14.57 Use this space for
June 2022, Unit 4, Q16(c) (1 mark): Write an ionic equation to show why aqueous solutions of ammonium chloride are acidic. State symbols are not required. (1)
January 2022, Unit 4, Q6 (1 mark): At 25 °C, the pH of pure water is 7.00 and at 100 °C, the pH of pure water is 6.14. What can be deduced from this information? Enthalpy change of Concentration
January 2022, Unit 4, Q12(b) (1 mark): The rate equation for the reaction between propanone and iodine in acidic conditions is rate = k[H+][CH3COCH3] The reaction was carried out at two different pH
January 2022, Unit 4, Q19(a)(ii) (2 marks): Explain why the reaction between propanal and HCN in the absence of KCN is very slow, referring to the value of Ka. No calculation is required. [For HCN, Ka = 4
January 2022, Unit 4, Q20(a)(i) (1 mark): Write the equation for the acid dissociation constant, Ka, of hexanoic acid. (1)
January 2022, Unit 4, Q20(a)(ii) (4 marks): Calculate the pH of a 0.100 mol dm–3 solution of hexanoic acid. [pKa of hexanoic acid = 4.88] (4)
January 2022, Unit 4, Q22(e)(i) (2 marks): Write an equation for the production of this fertiliser. State symbols are not required. (2)
January 2022, Unit 4, Q22(e)(ii) (1 mark): Write an ionic equation to show that ammonium ions are acidic in aqueous solution. State symbols are not required. (1)
October 2021, Unit 4, Q8 (1 mark): When ethanoic acid and chloroethanoic acid are mixed, an equilibrium is set up. CH3COOH + CH2ClCOOH  CH3COOH2 + + CH2ClCOO− The Brønsted-Lowry acids in this eq
October 2021, Unit 4, Q9 (1 mark): What is the pH of 0.010 mol dm−3 aqueous calcium hydroxide, Ca(OH)2(aq)? [pKw = 14] A 11.7 B 12.0 C 12.3 D 13.3 Use this space for any rough working. Anything y
October 2021, Unit 4, Q20(a)(i) (1 mark): Write the equation for the dissociation of the hydrogensulfate ion in aqueous solution. State symbols are not required. (1)
October 2021, Unit 4, Q20(a)(ii) (5 marks): A solution of sodium hydrogensulfate has pH = 1.13 Calculate the concentration of this solution, in g dm−3. − = 1.92] [pKa of HSO4 (5)
October 2021, Unit 4, Q20(a)(iii) (2 marks): State the assumptions you have used in (a)(ii). (2)
January 2021, Unit 4, Q9(a) (1 mark): What is the pH of a 0.100 mol dm−3 solution of chloroethanoic acid? (1) A 0.27 B 1.95 C 2.90 D 3.90
January 2021, Unit 4, Q9(b) (1 mark): Which is the acid-conjugate base pair in the reaction between ethanoic acid and chloroethanoic acid? (1) Acid Conjugate base CH3COO− A CH3COOH + B CH3COOH CH3CO
January 2021, Unit 4, Q10(c) (1 mark): What is the approximate pH of an ammonium chloride solution? (1) A 2.0 B 5.8 C 9.7 D 11.3
January 2021, Unit 4, Q18(b)(ii) (1 mark): The pH of a 0.10 mol dm−3 solution of sulfuric acid at 25 °C is 0.97. Calculate the concentration of hydrogen ions, in mol dm−3, in this solution. (1)
January 2021, Unit 4, Q18(b)(iii) (2 marks): In an aqueous solution of sulfuric acid, the following equilibria exist. H2SO4(aq) + H2O(l) U H3O+(aq) + HSO4 −(aq) Ka very large −(aq) + H2O(l) U H3O+(aq) + SO
October 2020, Unit 4, Q18(a)(i) (2 marks): A solution of hydrochloric acid has a pH of 1.125. Calculate the concentration of hydrogen ions and of hydroxide ions, in mol dm−3, in this solution. [Kw = 1.00
October 2020, Unit 4, Q18(a)(ii) (4 marks): Calculate the volume of water which must be added to 25.0 cm3 of this solution to increase the pH from 1.125 to 1.500, giving your answer to an appropriate numb
October 2020, Unit 4, Q18(b)(i) (2 marks): Identify the acids and bases in the spaces below Equation 2, linking the acid-base conjugate pairs. (2)
October 2020, Unit 4, Q18(b)(ii) (2 marks): Explain why very little dissociation of the H2PO4 phosphoric(V) acid. (2)
October 2020, Unit 4, Q18(c)(i) (1 mark): Using Equation 1, write an expression for the acid dissociation constant, Ka1 , for phosphoric(V) acid, H3PO4 . (1)
October 2020, Unit 4, Q18(c)(ii) (4 marks): A 0.500 mol dm−3 solution of phosphoric(V) acid has a pH of 1.20. Calculate the value of Ka1 , stating its units. Assume that there is no further dissociation o
January 2020, Unit 4, Q10 (1 mark): How are 20 cm3 of 0.05 mol dm–3 H2SO4(aq) and 20 cm3 of 0.10 mol dm–3 CH3COOH(aq) alike? Both solutions A have the same pH B have the same total concentration o
January 2020, Unit 4, Q12 (1 mark): A solution of sodium hydroxide of concentration 0.0080 mol dm–3 has a pH A between 7 and 9 B between 9 and 11 C between 11 and 13 D above 13 Use this space for
January 2020, Unit 4, Q20(a)(iii) (3 marks): The students used 25.0 cm3 of sodium hydroxide solution with concentration 0.980 mol dm–3 for the hydrolysis. Calculate the pH of the reaction mixture when the

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