OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), November 2021: Question 5

1 mark · Medium difficulty · Multiple Choice

Calculate the minimum mass of magnesium required to reduce 11.4 g of chromium(III) oxide.

Practise this question

Question

Multiple choice question 5 stating that chromium(III) oxide, Cr2O3, is reduced to chromium by heating with magnesium, asking for the minimum mass of Mg required to reduce 11.4 g of chromium(III) oxide. Four options are given: A 0.61 g, B 0.91 g, C 3.65 g, and D 5.47 g, with a box for the answer and 1 mark indicated.
Question text

5 Chromium(III) oxide, Cr2O3, is reduced to chromium by heating with magnesium.

What is the minimum mass of Mg required to reduce 11.4 g of chromium(III) oxide?

A 0.61 g

B 0.91 g

C 3.65 g

D 5.47 g

Your answer [1]

Mark scheme

Show the mark scheme Mark scheme table showing question number 5 with the correct answer D, 1 mark, and assessment objective AO2.6.

5 D 1 AO2.6

How to answer it

Mass Calculations in Metal Extraction

What this question tests

This question assesses your ability to apply quantitative chemistry skills (AO2.6), specifically writing balanced chemical equations, calculating moles from mass using molar masses, and using stoichiometric reacting ratios to find the mass of an unknown reactant.

Question 5 Multiple Choice

Full Worked Solution & Breakdown

✅ Correct Answer: D (5.47 g)

Option D is the correct minimum mass of magnesium required to completely reduce 11.4 g of chromium(III) oxide.

💡 Key Knowledge

  • Molar Mass (M): Cr₂O₃ = (52.0 × 2) + (16.0 × 3) = 152.0 g mol⁻¹
  • Molar Mass (M): Mg = 24.3 g mol⁻¹
  • Balanced Equation: 3Mg + Cr₂O₃ → 3MgO + 2Cr

🧠 Exam Technique

Always start stoichiometric multi-step calculations by balancing the reduction equation first. Forgetting to balance the equation is the number one cause of lost marks in reacting mass calculations.

❌ Common Errors

  • Using an incorrect mole ratio (e.g., using a 1:1 ratio instead of 3:1).
  • Inverting the molar mass or miscalculating the Mr of Cr₂O₃ (e.g., forgetting to multiply the atomic mass of chromium by 2).
  • Rounding intermediate values too early, leading to rounding errors in the final multiple-choice selection.

📐 Step-by-Step Calculation Guide

  1. Step 1: Write the balanced symbol equation
    3Mg + Cr₂O₃ → 3MgO + 2Cr
    Notice the 3:1 reacting ratio between Mg and Cr₂O₃.
  2. Step 2: Calculate moles of chromium(III) oxide
    Molar mass of Cr₂O₃ = 152.0 g mol⁻¹
    Moles of Cr₂O₃ = Mass ÷ Molar Mass = 11.4 g ÷ 152.0 g mol⁻¹ = 0.075 mol
  3. Step 3: Use the stoichiometric ratio to find moles of Mg
    From the equation, 1 mole of Cr₂O₃ reacts with 3 moles of Mg.
    Moles of Mg needed = 0.075 mol × 3 = 0.225 mol
  4. Step 4: Calculate the final mass of magnesium
    Mass = Moles × Molar Mass (Mg = 24.3 g mol⁻¹)
    Mass of Mg = 0.225 mol × 24.3 g mol⁻¹ = 5.4675 g
    Rounding appropriately gives 5.47 g (matching option D).
Examiner Note: This is an AO2.6 assessment objective question testing application of knowledge. Students who selected distractors like B (0.91 g) typically failed to multiply the moles of chromium(III) oxide by 3 to account for the stoichiometric ratio.

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), November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.