OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), June 2017: Question 3

1 mark · Easy difficulty · Multiple Choice

Determine the oxidation number of manganese in potassium manganate(VI), K2MnO4.

Practise this question

Question

Multiple choice question 3 asking for the oxidation number of Mn in K2MnO4, with four options: A (+4), B (+5), C (+6), and D (+7), along with an answer box.
Question text

3 What is the oxidation number of Mn in K2MnO4?

A +4

B +5

C +6

D +7

Your answer [1]

Mark scheme

Show the mark scheme Mark scheme indicating the correct answer is C, with 1 mark awarded, and allowing +6 in the answer box.

3 C 1 ALLOW +6 in the box

How to answer it

Determining the Oxidation Number of Manganese

What this question tests

This multiple-choice question tests your ability to apply rules for assigning oxidation numbers to elements in neutral ionic compounds, specifically focusing on transition metal complexes where the central metal atom exhibits variable oxidation states (such as manganese in manganate(VI) versus permanganate).

Question 3 • Multiple Choice [1 Mark]

Find the oxidation number of Mn in K₂MnO₄

✅ Correct Answer

The correct option is C (+6).

Mark scheme note: Award 1 mark for selecting C (or writing +6 in the answer box).

💡 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 and fluorine compounds).
  • The sum of all oxidation numbers in a neutral compound must equal 0 .

🧠 Exam Technique

Set up a simple algebraic equation using known values to solve for the unknown transition metal oxidation state. Always include the explicit sign (+ or -) in your final answer, as omitting the plus sign can lose you marks in unstructured questions.

📐 Step-by-Step Calculation

  1. Identify the formula: K₂MnO₄
  2. Assign knowns: K = +1 and O = -2
  3. Set up the equation: (2 × +1) + Mn + (4 × -2) = 0
  4. Simplify: +2 + Mn - 8 = 0
  5. Solve for Mn: Mn - 6 = 0 so Mn = +6

❌ Common Errors

  • Confusing with Permanganate: Many students automatically guess +7 because they confuse K₂MnO₄ (manganate(VI)) with potassium permanganate ( KMnO₄ ).
  • Stoichiometry Slip: Forgetting to multiply the oxidation number of potassium by 2 ( 2 × +1 ) due to the subscript in the formula.

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 2017. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.