OCR A-Level Chemistry AS Breadth in chemistry (01), November 2020: Question 7

1 mark · Medium difficulty · Multiple Choice

Identify the correct formula of the ferrate(VI) ion given that potassium ferrate(VI) contains two potassium ions for every ferrate(VI) ion.

Practise this question

Question

Multiple choice question 7 stating that potassium ferrate(VI) contains two potassium ions for every ferrate(VI) ion, asking for the formula of the ferrate(VI) ion with four options: A FeO3 2-, B FeO4 2-, C FeO5 2-, and D FeO6 2-.
Question text

7 Potassium ferrate(VI) contains two potassium ions for every ferrate(VI) ion.

What is the formula of the ferrate(VI) ion?

A FeO 2−

B FeO 2−

C FeO 2−

D FeO 2−

Your answer [1]

Mark scheme

Show the mark scheme Mark scheme indicating the correct answer is B for question 7, worth 1 mark.

7 B 1 1.2

How to answer it

Determining the Formula of Potassium Ferrate(VI)

What this question tests

This question assesses your understanding of oxidation states (oxidation numbers), ionic compound formulae, and how to deduce the charge and stoichiometry of complex anions using Roman numerals and overall neutral compound neutrality.

Question 7

Full Worked Solution & Examiner Analysis

✅ Correct Answer

B ( FeO₄²⁻ )

Potassium ferrate(VI) has the chemical formula K₂FeO₄ , meaning the ferrate(VI) ion itself is FeO₄²⁻ .

💡 Key Knowledge

  • Oxidation Numbers: Roman numerals in brackets indicate the oxidation state of the preceding element (here, Iron is in the +6 oxidation state).
  • Potassium Ions: Potassium is in Group 1, so it always forms a K⁺ ion in compounds.
  • Compound Neutrality: Ionic compounds have an overall charge of zero. Two K⁺ ions mean the counter-ion must carry a 2⁻ charge.

🧠 Exam Technique

Use simultaneous logic checks:

  1. Calculate iron's oxidation state in the given options where oxygen is -2 .
  2. Option B: Fe + 4(-2) = -2 ⇒ Fe - 8 = -2 ⇒ Fe = +6. This matches the roman numeral (VI)!
  3. Verify ion charges against the stem: two K⁺ ions balance one 2⁻ anion.

❌ Common Errors

  • Miscalculating oxidation states: Confusing the overall ion charge with the individual oxidation state of iron.
  • Ignoring stoichiometry clues: The stem explicitly states there are two potassium ions for every ferrate(VI) ion, meaning the anion must be divalent ( 2⁻ ). Options A and C have incorrect oxygen counts or charges that do not yield iron(VI).
Mark Scheme Allocation: 1 mark awarded for selecting option B. Examiner reports indicate this tests fundamental nomenclature and formula derivation skills required early in AS Chemistry.

Topics

Module 2: Foundations in chemistry · Module 5: Physical chemistry and transition elements · 5.3 Transition elements · 2.1 Atoms and reactions

Question and mark scheme from the OCR A-Level Chemistry examination, AS Breadth in chemistry (01), November 2020. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.