OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), June 2024: Question 13

1 mark · Medium difficulty · Multiple Choice

Identify the formula of a salt based on its test results with sodium hydroxide and barium nitrate solutions.

Practise this question

Question

A multiple-choice question showing a table with qualitative test results: reaction with NaOH(aq) gives a green precipitate, and reaction with Ba(NO3)2(aq) gives a white precipitate. Four options are given: A, CuCl2; B, CuSO4; C, FeCl2; D, FeSO4. A box for the answer is provided at the bottom with a mark allocation of [1].
Question text

13 A student analyses a solution of a salt.

The results are shown below.

Test Observation

Reaction with NaOH(aq) Green precipitate

Reaction with Ba(NO3)2(aq) White precipitate

What is the formula of the salt?

A CuCl2

B CuSO4

C FeCl2

D FeSO4

Your answer [1]

Mark scheme

Show the mark scheme The mark scheme indicates that the correct answer for question 13 is D, worth 1 mark.

13 D 1

How to answer it

Qualitative Analysis: Identifying Ions in Salt Solutions

OCR A-Level Chemistry • Multiple Choice Question

What this question tests

This question assesses your recall and application of qualitative test observations for transition metal / aqueous cations using sodium hydroxide solution, and sulfate/halide anion tests using acidified barium chloride or barium nitrate. You must combine clues logically to deduce the formula of an unknown salt.

Question 13 Analysis

Identifying the Cation and Anion Step-by-Step

✅ Correct Answer: D (FeSO₄)

The correct option is D because iron(II) sulfate yields a green precipitate with sodium hydroxide and a white precipitate with barium nitrate.

💡 Key Knowledge: Cation Testing with NaOH(aq)

  • Fe²⁺(aq) + 2OH⁻(aq) → Fe(OH)₂(s) produces a green precipitate (which may darken on standing in air due to oxidation to Fe³⁺).
  • Fe³⁺(aq) produces a brown precipitate.
  • Cu²⁺(aq) produces a blue precipitate.

💡 Key Knowledge: Anion Testing with Ba²⁺(aq)

  • SO₄²⁻(aq) + Ba²⁺(aq) → BaSO₄(s) produces a dense white precipitate of barium sulfate.
  • Chloride ions ( Cl⁻ ) show no reaction with barium nitrate solution.

🧠 Exam Technique

Break the table into two independent parts. First, use the cation test to eliminate options (narrowing down to Fe²⁺), then use the anion test to confirm the counter-ion.

❌ Common Errors

Students frequently confuse transition metal colours: mixing up the blue precipitate of copper(II) with the green precipitate of iron(II), or confusing chloride tests (AgNO₃) with sulfate tests (Ba²⁺).

Mark Scheme Allocation: 1 mark awarded for selecting D.

Topics

Module 3: Periodic table and energy · Module 5: Physical chemistry and transition elements · Practical Activity Groups · 3.1 The periodic table · 5.3 Transition elements · PAG 4: Qualitative analysis of ions

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.