Practice question
Question
A 1.5 L sample of a gas at STP weighs 2.1 g and contains only carbon and hydrogen. If it produces 3.52 g of CO₂ on burning, what is its molecular formula? (Atomic masses: C = 12, H = 1)
Explanation
Moles of gas = 1.5 / 22.4 ≈ 0.067 mol; molar mass = 2.1 / 0.067 ≈ 31.34 g/mol. Mass of C = (12/44) × 3.52 ≈ 0.96 g; moles of C = 0.96/12 = 0.08. C per molecule ≈ 1; H mass = 2.1 − 0.96 = 1.14 g; H ≈ 6 (adjusted). Molecular formula = C₂H₆ (molar mass 30, close to 31.34).
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