OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), June 2022: Question 6

1 mark · Medium difficulty · Multiple Choice

Determine the number of Fe2+ and Fe3+ ions in one formula unit of Prussian blue, C18Fe7N18.

Practise this question

Question

Multiple choice question asking for the numbers of Fe2+ and Fe3+ ions in one formula unit of Prussian blue, C18Fe7N18, containing Fe2+, Fe3+, and CN- ions. Four options are provided: A, B, C, and D, each with different combinations of Fe2+ and Fe3+ ion numbers. An answer box is provided at the bottom with a mark allocation of [1].
Question text

6 Prussian blue, C Fe N , is a deep blue pigment containing Fe2+, Fe3+ and CN− ions.

18 7 18

What are the numbers of Fe2+ and Fe3+ ions in one formula unit of C Fe N ?

18 7 18

A 2 Fe2+ and 5 Fe3+

B 3 Fe2+ and 4 Fe3+

C 4 Fe2+ and 3 Fe3+

D 5 Fe2+ and 2 Fe3+

Your answer [1]

Mark scheme

Show the mark scheme Mark scheme indicating the correct answer for question 6 is B.

6 B 1 1.2

How to answer it

Prussian Blue Ion Stoichiometry

What this question tests

This question assesses your ability to apply the principle of charge balance (neutrality) to complex ionic formulas, combining knowledge of transition metal oxidation states (Fe²⁺ and Fe³⁺) with polyatomic ligand charges (CN⁻).

Question 6 Multiple Choice

Determining Iron Oxidation States in C₁₈Fe₇N₁₈

✅ Correct Answer: B (3 Fe²⁺ and 4 Fe³⁺)

The correct option is B. One formula unit of C₁₈Fe₇N₁₈ contains 3 moles of Fe²⁺ and 4 moles of Fe³⁺ (totalling 7 iron atoms).

💡 Key Knowledge

  • The cyanide ion ( CN⁻ ) always carries a single negative charge of -1 .
  • Since there are 18 nitrogen atoms and 18 carbon atoms arranged as 18 CN⁻ units, the total negative charge from the ligands is 18 × (-1) = -18 .
  • A neutral formula unit must have a total positive charge of +18 to balance the -18 charge.

🧠 Exam Technique

Instead of guessing combinations, test each multiple-choice option mathematically by calculating the total positive charge for the stated number of Fe²⁺ and Fe³⁺ ions and checking if it equals +18 .

❌ Common Errors

Students often misread the total number of iron atoms given in the formula ( Fe₇ ) or incorrectly assume that all iron ions in complex pigments must exist in the same oxidation state.

📐 Step-by-Step Calculation Proof for Option B

  1. Total negative charge: 18 × (CN⁻) = 18 × (-1) = -18 .
  2. Test Option B (3 Fe²⁺ and 4 Fe³⁺):
    • Positive charge from Fe²⁺ = 3 × (+2) = +6
    • Positive charge from Fe³⁺ = 4 × (+3) = +12
    • Total positive charge = +6 + +12 = +18 .
  3. Conclusion: The total positive charge (+18) exactly balances the total negative charge (-18), confirming Option B is correct.
Mark Awarded: 1 mark for selecting B.

Topics

Module 2: Foundations in chemistry · 2.1 Atoms and reactions

Question and mark scheme from the OCR A-Level Chemistry examination, Periodic table, elements and physical chemistry (01), June 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.