OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), June 2022: Question 5

✅ Correct Answer: B (O₂)

The correct option is B. Calculating the molar mass yields approximately 32.0 g mol⁻¹ , which corresponds to diatomic oxygen, O₂.

💡 Key Knowledge

• Avogadro's Constant (Nₐ): 6.02 × 10²³ particles in one mole.
• Molar Mass (M): The mass of 1 mole of a substance, expressed in g mol⁻¹ . Numerically equal to relative molecular mass (Mᵣ).
• Atomic Masses: H = 1.0, C = 12.0, O = 16.0, S = 32.1, I = 126.9.

🧠 Exam Technique

For multiple-choice questions involving calculations, you do not need to calculate the exact molar mass with extreme precision before looking at the options. Estimate or calculate roughly, then match against the molar masses of the given choices (A: CH₄ = 16, B: O₂ = 32, C: SO₂ = 64, D: I₂ = 253.8).

📐 Step-by-Step Calculation

1. Find the mass of 1 molecule:
   Mass of 1 molecule = (2.125 × 10⁻²² g) / 4 = 5.3125 × 10⁻²³ g
2. Scale up to 1 mole (multiply by Avogadro's constant):
   Molar Mass = 5.3125 × 10⁻²³ × 6.022 × 10²³ = 32.0 g mol⁻¹
3. Match with options:
   O₂ has an Mᵣ of (16.0 × 2) = 32.0 .

❌ Common Errors & Traps

• Forgetting to divide by 4: Treating the given mass as the mass of 1 molecule instead of 4 molecules.
• Inverting Avogadro's constant: Dividing by 6.02 × 10²³ instead of multiplying when scaling up from a single molecule to a mole.
• Confusing atomic and molecular mass: Selecting atomic oxygen (O = 16) instead of molecular oxygen (O₂ = 32).