2020 O Level Chem P1 Q10 — Stoichiometry / Mole Calculations Solution

Answer: B (65). First find the moles of carbon dioxide produced. At room temperature and pressure one mole of gas occupies 24 dm³, so moles of CO₂ \(= 1.2/24 = 0.05\) mol. From the equation \(\text{XCO}_3 \to \text{XO} + \text{CO}_2\), the ratio of XCO₃ to CO₂ is 1 : 1, so moles of XCO₃ \(= 0.05\) mol.

The relative formula mass of XCO₃ is mass divided by moles: \(6.25/0.05 = 125\). The carbonate group CO₃ contributes \(12 + 3(16) = 60\), so the relative atomic mass of X is \(125 - 60 = 65\).

That gives \(A_r = 65\), which corresponds to zinc, so the answer is B. Common error: forgetting to subtract the mass of the whole carbonate group, or dividing by 22.4 instead of using the 24 dm³ molar volume at r.t.p.