Edexcel A-Level Chemistry Paper 3, June 2019: Question 1

6 marks · Medium difficulty · Short Open Response

State the definition of a Bronsted-Lowry acid, identify conjugate pairs, define pH, calculate hydrogen ion concentration from pH, and explain why a very dilute strong acid does not have an alkaline or neutral pH.

Practise this question

Question

An Edexcel A-Level Chemistry question about acids and bases containing five parts (a) to (e). Part (a) asks to state what is meant by a Brønsted-Lowry acid (1 mark). Part (b) asks to identify acid-base conjugate pairs in the reaction CH3COOH + HCOOH -> CH3COOH2+ + HCOO- (1 mark). Part (c) asks to write the expression that defines pH (1 mark). Part (d) asks to calculate the concentration of hydrogen ions in mol dm-3 for a pH of 2.76 (1 mark). Part (e) asks to explain why the pH of a 1 x 10^-8 mol dm-3 solution of nitric acid is not 8, given Kw = 1.00 x 10^-14 mol2 dm-6 (2 marks).
Question text

1 This question is about acids and bases.

(a) State what is meant by a Brønsted-Lowry acid.

(1)

(b) Identify the acid-base conjugate pairs in this reaction.

(1)

CH COOH + HCOOH o CH COOH+ + HCOO–

33 2

(c) Write the expression that defines the pH of a solution.

(1)

(d) Calculate the concentration of hydrogen ions, in mol dm–3, in a solution with a

pH of 2.76

(1)

(e) Explain why the pH of a 1 × 10–8 mol dm–3 solution of nitric acid, HNO , is not 8.

[Ionic product of water, K = 1.00 × 10–14 mol2 dm–6]

w

(2)

*P58308A0232*(Total for Question 1 = 6 marks)

Mark scheme

Show the mark scheme The mark scheme for the question providing acceptable answers and additional guidance for each of the five parts. Part (a) accepts proton donor. Part (b) accepts correct identification and linking of acid-base conjugate pairs. Part (c) accepts -(pH =) -log[H+(aq)] or -log[H3O+(aq)]. Part (d) accepts 1.74 x 10^-3 or 0.00174 (mol dm-3). Part (e) awards 1 mark for stating that [H+] from water is 1.0 x 10^-7 mol dm-3 and 1 mark for explaining that the total [H+] is greater than 1 x 10^-8 mol dm-3 or that the concentration of hydrogen ions from water cannot be ignored.

How to answer it

Acids, Bases, and pH Calculations Study Guide

What this question tests

This question assesses fundamental acid-base equilibria theory, including the Brønsted-Lowry definition, identification of conjugate acid-base pairs, logarithmic pH expressions, inverse log calculations for hydrogen ion concentrations, and advanced understanding of the ionic product of water ( Kw ) in extremely dilute strong acid solutions.

Part (a)

Brønsted-Lowry Acid Definition

State what is meant by a Brønsted-Lowry acid. (1 mark)

✅ Correct Answer

A proton donor (or gives away / releases / loses protons / H+ ions).

❌ Common Errors

Writing H3O+ donor instead of proton/H+ donor. Examiners strictly penalise inaccurate chemical terminology here.

Mark breakdown: 1 mark for stating proton donor or equivalent wording.
Part (b)

Conjugate Acid-Base Pairs

Identify the acid-base conjugate pairs in this reaction: CH3COOH + HCOOH ⇌ CH3COOH2+ + HCOO- (1 mark)

✅ Correct Answer

Pairs must be clearly identified and linked (e.g., via labels, lines, or clear annotations):

  • Pair 1: CH3COOH (base) and CH3COOH2+ (acid) — Note: In this unusual proton transfer direction, ethanoic acid acts as the base accepting a proton from methanoic acid!
  • Pair 2: HCOOH (acid) and HCOO- (base)

🧠 Exam Technique

Draw clear connecting lines underneath the equation between the corresponding species, or label them explicitly as Pair 1 / Pair 2 or Acid 1 / Base 1.

Mark breakdown: 1 mark for correct identification and linking of both pairs.
Part (c)

Defining pH

Write the expression that defines the pH of a solution. (1 mark)

✅ Correct Answer

pH = -log[H+(aq)] or pH = -log[H3O+(aq)]

❌ Common Errors

Omitting the minus sign, writing -log conc H+ without square brackets, or missing brackets around the concentration term entirely.

Mark breakdown: 1 mark for the correct mathematical expression including square brackets and the negative sign.
Part (d)

Calculating Hydrogen Ion Concentration

Calculate the concentration of hydrogen ions, in mol dm-3, in a solution with a pH of 2.76. (1 mark)

📐 Step-by-Step Calculation

  1. Recall the inverse formula: [H+] = 10-pH
  2. Substitute values: [H+] = 10-2.76
  3. Evaluate: = 0.0017378... mol dm-3 (Acceptable variations: 1.74 × 10-3 , 1.7378 × 10-3 , 0.00174 , or 0.0017 ).
Mark breakdown: 1 mark for a correct numerical value calculated to 2 or more significant figures. Units are ignored even if incorrect.
Part (e)

Dilute Strong Acid pH Anomaly

Explain why the pH of a 1 × 10-8 mol dm-3 solution of nitric acid, HNO3 , is not 8. (2 marks)
[Ionic product of water, Kw = 1.00 × 10-14 mol2 dm-6 ]

💡 Key Knowledge

Acids cannot have a pH greater than 7 at standard conditions. In extremely dilute solutions of strong acids, the dissociation of water itself contributes a significant concentration of H+ ions that cannot be ignored ( [H+]water = 1.0 × 10-7 mol dm-3 ).

✅ Marking Points (Any 2)

  • [H+] from water is 1.0 × 10-7 mol dm-3 .
  • Total [H+] is greater than 1.0 × 10-7 (specifically 1.1 × 10-7 mol dm-3 ).
  • pH cannot be more than 7 / cannot be alkaline for an acid.
  • Concentration of H+ from water is non-negligible / cannot be ignored.

❌ Common Pitfalls

Students often lose the second mark by assuming 10-8 implies a pH of 8 without factoring in the auto-ionisation of water. Remember: an acid solution can never be alkaline (pH > 7).

Mark breakdown: 2 marks total (1 mark per valid explanatory point referenced from the mark scheme).

Topics

Physical Chemistry · Topic 12: Acid-base Equilibria

Question and mark scheme from the Edexcel A-Level Chemistry examination, Paper 3, June 2019. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.