OCR A-Level Chemistry AS Breadth in chemistry (01), June 2022: Question 7

✅ Correct Answer: C (45.5 g)

The correct option is C. Following the stoichiometric ratio of the balanced equation yields exactly 45.5 g of lead(II) sulfate required to produce 2.40 g of oxygen gas.

💡 Key Knowledge

• Moles formula: Moles = Mass / Molar Mass (n = m / M)
• Reacting ratios: Coefficients in balanced equations dictate the molar proportions. Here, 2 moles of PbSO₄ produce 1 mole of O₂ .
• Molar mass of O₂: 16.0 × 2 = 32.0 g mol⁻¹

📐 Step-by-Step Calculation

1. Calculate moles of O₂ formed:
   Moles = 2.40 g / 32.0 g mol⁻¹ = 0.0750 mol
2. Use the stoichiometric ratio from the equation:
   From 2PbSO₄(s) → 2PbSO₃(s) + O₂ , the ratio of PbSO₄ to O₂ is 2 : 1.
   Moles of PbSO₄ = 0.0750 mol × 2 = 0.150 mol
3. Calculate the mass of PbSO₄ heated:
   Mass = Moles × Molar Mass
   Mass = 0.150 mol × 303.3 g mol⁻¹ = 45.495 g
4. Round to appropriate significant figures:
   The input data (2.40 g) is given to 3 significant figures, so round to 3 sig fig: 45.5 g

❌ Common Errors & Calculation Traps

• Ignoring the 2:1 stoichiometry ratio: Students often assume a 1:1 ratio between reactant and product if they rush reading the balanced equation, leading to half the correct moles and selecting Option B (30.3 g).
• Using incorrect molar mass for O₂: Using atomic mass (16.0 g mol⁻¹) instead of molecular mass (32.0 g mol⁻¹) for oxygen gas will throw off the entire calculation by a factor of 2.
• Inverted ratio trap: Multiplying when you should divide, or vice versa, during ratio scaling. Always write out units clearly to double-check your dimensional analysis.