AQA A-Level Chemistry Paper 3, 2019: Question 25

1 mark · Medium difficulty · Multiple Choice

Identify a suitable drying agent to dry a liquid sample of pentanoic acid from a list of options.

Practise this question

Question

Question 25 asks: 'A student is required to dry a liquid sample of pentanoic acid. Which drying agent is suitable?' followed by four multiple choice options: A Calcium oxide, B Calcium sulfate, C Potassium hydroxide, and D Potassium carbonate.
Question text

25 A student is required to dry a liquid sample of pentanoic acid.

Which drying agent is suitable?

[1 mark]

A Calcium oxide

B Calcium sulfate

C Potassium hydroxide

D Potassium carbonate

Mark scheme

Show the mark scheme Mark scheme for question 25 showing the correct answer as B with a value of 1 mark.

25 B 1

How to answer it

AQA A-Level Chemistry • Organic Practical Skills • Multiple Choice

Selecting a Drying Agent for a Carboxylic Acid

What this question tests

This question assesses practical organic chemistry knowledge (Required Practical 5/10): selecting an appropriate anhydrous inorganic salt to dry an organic liquid. Specifically, it tests your ability to identify chemical compatibility—a drying agent must absorb water without chemically reacting with the organic functional group (in this case, an acidic carboxylic acid).

Question 25 Analysis

Question: Which drying agent is suitable to dry a liquid sample of pentanoic acid?

✅ Correct Answer

B: Calcium sulfate

1 Mark awarded for option B

Anhydrous calcium sulfate ( CaSO₄ ) is a neutral, insoluble inorganic salt that readily forms a hydrate by absorbing dissolved water, without reacting with carboxylic acids.

💡 Key Knowledge: Criteria for Drying Agents

  • Must be inert: It cannot react with the organic product being dried.
  • Insoluble: Must remain solid in the organic liquid so it can be filtered or decanted away.
  • Common neutral drying agents: Anhydrous MgSO₄ , Na₂SO₄ , and CaSO₄ .
  • Nature of substrate: Pentanoic acid ( CH₃(CH₂)₃COOH ) contains an acidic carboxyl group ( -COOH ).

❌ Why the Distractors are Incorrect

  • A (Calcium oxide, CaO): A basic metal oxide. It reacts vigorously with pentanoic acid in an acid-base neutralization to form calcium pentanoate and water:
    2RCOOH + CaO → (RCOO)₂Ca + H₂O
  • C (Potassium hydroxide, KOH): A strong alkali. It reacts immediately with pentanoic acid to form potassium pentanoate salt:
    RCOOH + KOH → RCOOK + H₂O
  • D (Potassium carbonate, K₂CO₃): A basic salt that reacts with carboxylic acids with effervescence, producing CO₂ :
    2RCOOH + K₂CO₃ → 2RCOOK + H₂O + CO₂

🧠 Exam Technique: The Elimination Strategy

When asked to select a drying agent for an organic liquid:

  1. Classify the organic compound: Pentanoic acid is a carboxylic acid (Bronsted-Lowry acid).
  2. Check acid-base reactivity: Look at each option and eliminate any basic or alkaline substances ( O²⁻ , OH⁻ , CO₃²⁻ ).
  3. Select the neutral salt: Sulfates ( SO₄²⁻ ) of Group 2 metals or sodium are neutral and do not undergo acid-base reactions with weak acids.

Topics

Organic Chemistry · Required Practicals · 3.3.9 Carboxylic Acids and Derivatives · Required Practical 10: Preparation of a pure organic liquid

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