AQA A-Level Chemistry Paper 3, 2023: Question 18

1 mark · Easy difficulty · Multiple Choice

Determination of Ka for a Weak Acid This question involves calculating the acid dissociation constant (Ka) for a weak acid using the given concentration and pH. It tests knowledge of the relationship between Ka, [H⁺], and the initial acid concentration.

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Question

AQA A-Level Chemistry Paper 3, 2023: Question 18
Question text

18 A 0.100 mol dm–3 solution of a weak acid has pH = 2.50

What is the value of K for this acid, in mol dm–3?

a

[1 mark]

A 3.16 × 10–2

B 3.16 × 10–3

C 1.00 × 10–4

D 1.00 × 10–5

Mark scheme

Show the mark scheme Mark scheme for AQA A-Level Chemistry Paper 3, 2023: Question 18

1.00 × 10–4

18 C 1 (AO2)

How to answer it

Calculating the Ka for a Weak Acid

Question

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?

  • A: 3.16 × 10-2
  • B: 3.16 × 10-3
  • C: 1.00 × 10-4
  • D: 1.00 × 10-5

Correct Answer: C

The correct value of Ka is 1.00 × 10-4, calculated as follows:

Solution

  1. Determine [H+]: From the pH formula, [H+] = 10-pH = 10-2.50 = 3.16 × 10-3 mol dm-3.
  2. Set up the expression for Ka: Ka = ([H+]2) / ([HA] - [H+]).
  3. Substitute values:
    Ka = (3.16 × 10-3)2 / (0.100 - 3.16 × 10-3)
    Ka = 9.9856 × 10-6 / 0.09684
    Ka = 1.03 × 10-4 mol dm-3.
  4. Round to 3 significant figures: Ka = 1.00 × 10-4 mol dm-3.

Teacher Tip

Be careful when subtracting small values like [H+] from the initial concentration of the acid. For weak acids, [H+] is usually much smaller than [HA], so the approximation [HA] ≈ initial concentration is often valid unless stated otherwise.

Final Answer

C: 1.00 × 10-4 mol dm-3

Topics

Physical Chemistry · 3.1.12 Acids and Bases

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.