OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), November 2021: Question 10
1 mark · Medium difficulty · Multiple Choice
Calculate the pH of a 0.50 mol dm-3 aqueous solution of ethanoic acid given its pKa value at 25 °C.
Practise this questionQuestion
Question text
10 An aqueous solution of ethanoic acid, CH COOH, has a concentration of 0.50 mol dm–3.
pKa for CH3COOH = 4.76 at 25 °C.
What is the pH of the ethanoic acid solution at 25 °C?
A 2.53
B 2.68
C 4.91
D 5.06
Your answer [1]
Mark scheme
Show the mark scheme
10 A 1 AO2.6
How to answer it
Calculating the pH of a Weak Acid Solution
What this question tests
This question assesses your understanding of weak acid equilibria, conversion between pKa and Ka, manipulation of the weak acid dissociation expression, and logarithmic calculations to find pH to the correct precision.
Solution & Breakdown
✅ Correct Answer: A (2.53)
Option A is the correct pH value obtained by properly converting pKa to Ka, applying the weak acid approximation, and calculating −log[H⁺].
💡 Key Knowledge
- Weak acids only partially dissociate in aqueous solution: CH₃COOH ⇌ CH₃COO⁻ + H⁺
- Relationship formula: pKa = −log(Ka) , meaning Ka = 10⁻ᵖᴷᵃ
- Approximation used: [H⁺] = [CH₃COO⁻] and [CH₃COOH]equilibrium ≈ [CH₃COOH]initial
🧠 Exam Technique
For multiple-choice calculations, never guess. Fully write out your working margin-side. Watch out for distractors created by common mathematical errors (such as forgetting to square root or misusing base-10 logs).
❌ Common Errors
- Forgetting to take the square root of (Ka × [HA]) and directly taking −log of that product.
- Confusing pH with pKa or incorrectly rearranging Ka = [H⁺]² / [HA] .
📐 Step-by-Step Calculation
- Find Ka from pKa:
Ka = 10⁻⁴·⁷⁶ = 1.7378 × 10⁻⁵ mol dm⁻³ - Set up the weak acid equilibrium expression:
Ka = [H⁺]² / [HA] ⇒ [H⁺]² = Ka × [HA] - Substitute values and solve for [H⁺]:
[H⁺]² = (1.7378 × 10⁻⁵) × 0.50 = 8.689 × 10⁻⁶
[H⁺] = √(8.689 × 10⁻⁶) = 0.0029477 mol dm⁻³ - Calculate the pH:
pH = −log(0.0029477) = 2.5305...
Rounding to 2 decimal places (matching the precision of the given data) gives 2.53.
Topics
Module 5: Physical chemistry and transition elements · 5.1 Rates, equilibrium and pH
Question and mark scheme from the OCR A-Level Chemistry examination, Periodic table, elements and physical chemistry (01), November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.