OCR A-Level Chemistry AS Breadth in chemistry (01), November 2020: Question 13

1 mark · Easy difficulty · Multiple Choice

Calculate the numerical value of the equilibrium constant Kc for the reversible reaction between sulfur dioxide and oxygen given their equilibrium concentrations.

Practise this question

Question

A multiple choice question numbered 13 showing a reversible reaction: 2SO2(g) plus O2(g) in equilibrium with 2SO3(g). A table gives the equilibrium concentrations as 4.00 mol dm-3 for SO2, 2.40 mol dm-3 for O2, and 1.44 mol dm-3 for SO3. Four options are listed: A (0.0375), B (0.0540), C (0.150), and D (18.5), followed by an answer box and [1] mark allocation.
Question text

13 The reversible reaction below is in equilibrium.

2SO2(g) + O2(g) 2SO3(g)

The equilibrium concentrations are shown in the table.

Substance SO2(g) O2(g) SO3(g)

Equilibrium concentration

−3 4.00 2.40 1.44

/ mol dm

What is the numerical value of Kc?

A 0.0375

B 0.0540

C 0.150

D 18.5

Your answer [1]

Mark scheme

Show the mark scheme Mark scheme indicating the correct answer is B, with a mark of 1, also allowing 0.054(0).

13 B 1 1.2 ALLOW 0.054(0)

How to answer it

Calculating the Equilibrium Constant (Kc)

📌 What this question tests

This question assesses your ability to construct an equilibrium constant expression (Kc) from a balanced chemical equation, substitute equilibrium concentrations correctly, account for stoichiometric balancing numbers as powers, and calculate the final numerical value.

Question 13 Multiple Choice Analysis

Equilibrium Constant Calculation

✅ Correct Answer

B (0.0540)

The correct calculation yields approximately 0.0540 dm³ mol⁻¹ based on the equilibrium concentrations provided.

💡 Key Knowledge

  • Products are always on the numerator, reactants on the denominator.
  • Stoichiometric coefficients from the balanced equation become powers in the Kc expression.
  • Ensure you extract values designated for equilibrium concentrations, not initial amounts.

🧠 Exam Technique

  • Write out the general Kc expression explicitly before plugging in numbers to avoid transposition errors.
  • Double-check bracket positions and squaring functions on your calculator, particularly for terms with a coefficient of 2.

❌ Common Errors

  • Forgetting to square: Leaving out the squared term for SO₂(g) or SO₃(g) .
  • Inverting the expression: Putting reactants on top and products on the bottom.

📐 Step-by-Step Calculation Guide

  1. Write the balanced equation:
    2SO₂(g) + O₂(g) ⇌ 2SO₃(g)
  2. Construct the Kc expression:
    Kc = [SO₃]² / ([SO₂]² × [O₂])
  3. Substitute the equilibrium values from the table:
    [SO₂] = 4.00 mol dm⁻³
    [O₂] = 2.40 mol dm⁻³
    [SO₃] = 1.44 mol dm⁻³
  4. Perform the math:
    Numerator: 1.44² = 2.0736
    Denominator: 4.00² × 2.40 = 16.0 × 2.40 = 38.4
    Kc = 2.0736 / 38.4 = 0.0540 mol⁻¹ dm³
Mark Scheme Note: 1 mark awarded for selecting B. Accept answers rounded to 2 or 3 significant figures (e.g., 0.054 ).

Topics

Module 5: Physical chemistry and transition elements · 5.1 Rates, equilibrium and pH

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.