OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), June 2019: Question 3

1 mark · Medium difficulty · Multiple Choice

Identify the possible formula of a gas given that one molecule has a mass of 2.658 x 10^-23 g.

Practise this question

Question

Multiple choice question 3 asks to identify the formula of a gas given the mass of one molecule is 2.658 x 10^-23 g. Four options are provided: A, CH4; B, O2; C, SO2; D, SO3. An answer box is shown at the bottom with 1 mark indicated.
Question text

3 One molecule of a gas has a mass of 2.658 × 10−23 g.

What is a possible formula of the gas?

A CH4

B O2

C SO2

D SO3

Your answer [1]

Mark scheme

Show the mark scheme The mark scheme indicates that the correct answer for question 3 is A, carrying 1 mark under assessment objective AO2.2.

3 A 1 AO2.2

How to answer it

Calculating the Relative Molecular Mass of a Gas Molecule

What this question tests: This question assesses your understanding of the Avogadro constant, molar mass, and how to scale the mass of a single molecule up to find the relative molecular mass (Mr) of a substance.
Question 3 Multiple Choice

Part (3) - Identifying the Gas Formula

✅ Correct Answer

A (CH₄)

The calculated relative molecular mass is 16.0, which corresponds to methane (CH₄).

💡 Key Knowledge

  • Avogadro Constant (N_A): Approximately 6.022 × 10²³ mol⁻¹. This represents the number of particles in one mole.
  • Molar Mass relationship: The mass of one mole of a substance (in g mol⁻¹) is numerically equal to its relative molecular mass (Mr).
  • To go from the mass of one molecule to the mass of one mole, multiply by the Avogadro constant.

🧠 Exam Technique

Do not panic when given extremely small numbers with negative powers of 10. Recognise that scaling up a single molecular mass to a molar mass always requires multiplying by the Avogadro constant.

❌ Common Errors

  • Dividing instead of multiplying: Dividing the mass of a molecule by N_A results in an infinitesimally small number rather than a molar mass.
  • Incorrect Mr calculation: Confusing the mass of the single molecule with the Mr directly without scaling up to a mole.

📐 Step-by-Step Calculation

  1. Step 1: Understand the scale. Mass of 1 molecule = 2.658 × 10⁻²³ g . One mole contains 6.022 × 10²³ molecules.
  2. Step 2: Calculate the molar mass. Molar mass = (Mass of 1 molecule) × (Avogadro constant)
    Molar mass = (2.658 × 10⁻²³ g) × (6.022 × 10²³ mol⁻¹) = 16.0 g mol⁻¹ .
  3. Step 3: Match with the options. Find the option with Mr = 16.0:
    • CH₄ = 12.0 + (4 × 1.0) = 16.0 (Matches A)
    • O₂ = (2 × 16.0) = 32.0 (Incorrect)
    • SO₂ = 32.1 + (2 × 16.0) = 64.1 (Incorrect)
    • SO₃ = 32.1 + (3 × 16.0) = 80.1 (Incorrect)
Mark breakdown: [1 mark] awarded for selecting option A (AO2.2 - applying chemical knowledge and calculation skills).

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 2019. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.