OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), June 2024: Question 12

1 mark · Easy difficulty · Multiple Choice

Identify which solution can be added to ethanoic acid to make a buffer solution.

Practise this question

Question

Multiple choice question 12 asking which solution can be added to CH3COOH(aq) to make a buffer solution. The options are A: CH3COONa(aq), B: HCOOH(aq), C: HCl(aq), and D: NaCl(aq).
Question text

12 Which solution can be added to CH3COOH(aq) to make a buffer solution?

A CH3COONa(aq)

B HCOOH(aq)

C HCl(aq)

D NaCl(aq)

Your answer [1]

Mark scheme

Show the mark scheme The mark scheme indicates that the correct answer for question 12 is A, worth 1 mark.

12 A 1

How to answer it

Making an Acidic Buffer Solution

What this question tests

This question assesses your understanding of the chemical composition required to form an acidic buffer solution. Specifically, it tests whether you can identify that a buffer mixture requires a weak acid combined with a salt containing its conjugate base (providing a ready source of conjugate base anions).

Question 12 Analysis

Identifying the correct buffer components

✅ Correct Answer: A

CH₃COONa(aq)

Adding sodium ethanoate provides a high concentration of ethanoate ions (CH₃COO⁻), which forms an acidic buffer system when mixed with the weak ethanoic acid (CH₃COOH).

💡 Key Knowledge

  • An acidic buffer consists of a weak acid and the salt of that weak acid (its conjugate base).
  • Ethanoic acid ( CH₃COOH ) is a weak monoprotic acid.
  • Sodium ethanoate ( CH₃COONa ) is a soluble salt that dissociates completely to supply CH₃COO⁻ ions.

🧠 Exam Technique

Scan multiple-choice options for a species sharing the same acid radical/anion as the starting weak acid, but paired with a metal cation (like Na⁺) to form a soluble salt.

❌ Common Errors

  • Choosing a strong acid like HCl (Option C) will not create a buffer; it suppresses weak acid dissociation further and leads to a strongly acidic solution, not a pH-stable buffer.
  • Mixing two different weak acids (Option B, HCOOH ) does not provide a conjugate acid-base pair.
Mark Scheme Award: 1 mark for selecting A.

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), June 2024. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.