OCR A-Level Chemistry AS Breadth in chemistry (01), June 2023: Question 3

1 mark · Medium difficulty · Multiple Choice

Calculate the atom economy of hydrogen for the industrial preparation of hydrogen from methane and steam.

Practise this question

Question

Multiple choice question asking for the atom economy of hydrogen from the equation CH4(g) + 2H2O(g) -> 4H2(g) + CO2(g), with options A (3.8%), B (4.3%), C (15.4%), and D (17.4%).
Question text

3 Hydrogen can be prepared industrially by the reaction of methane with steam.

The equation is shown below.

CH4(g) + 2H2O(g) 4H2(g) + CO2(g)

What is the atom economy of hydrogen for this process?

A 3.8%

B 4.3%

C 15.4%

D 17.4%

Your answer [1]

Mark scheme

Show the mark scheme Mark scheme indicating the correct answer is C with 1 mark.

3 C 1 AO1.2

How to answer it

Calculating Atom Economy in Industrial Processes

📌 What this question tests

This question assesses your understanding of quantitative chemistry, specifically the application of the atom economy formula to a balanced chemical equation. It tests your ability to account for stoichiometric coefficients (moles) when calculating total molar masses of reactants and desired products.

Question 3 (Multiple Choice)

Reaction of Methane with Steam

✅ Correct Answer: C (15.4%)

Option C is the correct choice based on applying the stoichiometric atom economy formula correctly.

💡 Key Knowledge

  • Atom Economy Formula: (Mass of desired product / Total mass of all reactants) × 100
  • Always multiply individual molar masses by their balancing numbers (stoichiometric coefficients) from the equation.

🧠 Exam Technique

For multiple-choice questions involving calculations, double-check whether you remembered to multiply the molar mass of water by 2 and hydrogen by 4 before dividing!

❌ Common Errors

  • Omitting the stoichiometric coefficients (e.g., using Mr of H₂ as 2 instead of 4 × 2 = 8).
  • Including all products in the denominator instead of all reactants.

📐 Step-by-Step Calculation

Equation: CH₄(g) + 2H₂O(g) → 4H₂(g) + CO₂(g)

  1. Find the molar mass of the desired product (H₂):
    Desired product is 4 moles of H₂.
    Mass = 4 × (2 × 1.0) = 8.0 g mol⁻¹
  2. Find the total molar mass of all reactants:
    Reactants: 1 mole of CH₄ and 2 moles of H₂O.
    Mr of CH₄ = 12.0 + (4 × 1.0) = 16.0 g mol⁻¹
    Mr of H₂O = (2 × 1.0) + 16.0 = 18.0 g mol⁻¹
    Total reactant mass = 16.0 + (2 × 18.0) = 16.0 + 36.0 = 52.0 g mol⁻¹
  3. Calculate the Atom Economy:
    Atom Economy = (8.0 / 52.0) × 100 = 15.384...% = 15.4% (to 3 sig figs)
Mark Scheme Allocation: 1 mark available (AO1.2 - Demonstrate knowledge and understanding of scientific ideas). Answer: C.

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 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.