OCR A-Level Chemistry AS Breadth in chemistry (01), June 2018: Question 6

1 mark · Easy difficulty · Multiple Choice

Determine the oxidation number of iron in K2FeO4.

Practise this question

Question

Multiple choice question number 6 asking: What is the oxidation number of Fe in K2FeO4? Four options are provided: A, +4; B, +5; C, +6; and D, +7. An answer box is shown at the bottom left with [1] mark indicated on the right.
Question text

6 What is the oxidation number of Fe in K2FeO4?

A +4

B +5

C +6

D +7

Your answer [1]

Mark scheme

Show the mark scheme The mark scheme table indicates that question 6 has the correct answer C, with 1 mark awarded, and allows the notation +6.

6 C 1 ALLOW +6

How to answer it

Calculating Oxidation Numbers in Transition Metal Compounds

📋 What this question tests

This question assesses your ability to apply rules for assigning oxidation numbers to elements within a neutral ionic compound, specifically focusing on transition metal species with variable oxidation states like iron in potassium ferrate(VI) ( K₂FeO₄ ).

Question 6: Oxidation Number of Iron

Determining the oxidation state of Fe in K₂FeO₄

✅ Correct Answer

C (+6)

Mark awarded: 1 / 1 for selecting option C (or writing +6).

💡 Key Knowledge

  • Group 1 metals (like potassium, K) always have an oxidation number of +1 in compounds.
  • Oxygen almost always has an oxidation number of -2 (except in peroxides, superoxides, and fluorides).
  • The sum of oxidation numbers in a neutral compound equals 0.

🧠 Exam Technique

Set up a simple algebraic equation using the known values for K and O to solve for the unknown transition metal (Fe). Remember to account for the total number of atoms of each element present in the formula.

❌ Common Errors

  • Forgetting stoichiometry: Multiplying the oxidation number of K by 1 instead of 2 ( K₂ ).
  • Sign errors: Confusing the overall neutral charge ( = 0 ) with an ion charge.
  • Falling back on common stable ions like +2 or +3 for iron without calculating based on the given formula.

📐 Step-by-Step Calculation

  1. Identify known oxidation states: K = +1, O = -2. Let the oxidation number of Fe be x .
  2. Set up the equation for the neutral compound: (2 × (+1)) + x + (4 × (-2)) = 0
  3. Simplify the terms: (+2) + x + (-8) = 0
  4. Rearrange to solve for x: x - 6 = 0 → x = +6

Topics

Module 2: Foundations in chemistry · 2.1 Atoms and reactions

Question and mark scheme from the OCR A-Level Chemistry examination, AS Breadth in chemistry (01), June 2018. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.