Key Concepts: Balanced equations conserve mass. Amount of substance is measured in moles. Use $n = m/ M_r$ and mole ratios for reacting masses. Empirical and molecular formulae can be deduced from data.
Section A: Equations and Moles
1. Write balanced symbol equations (include state symbols). [4]
(a) Magnesium + hydrochloric acid → magnesium chloride + hydrogen
(b) Calcium carbonate → calcium oxide + carbon dioxide
2. Calculate $M_r$ for: H₂SO₄, Ca(OH)₂, (NH₄)₂SO₄. [3]
3. Calculate moles of: 9 g of water, 4.0 g of sodium hydroxide. [2]
Section B: Reacting Masses and Percentage Yield
4. 2H₂ + O₂ → 2H₂O. If 8 g of oxygen reacts, calculate the mass of water formed. [3]
5. The theoretical yield of a reaction is 10 g. The actual yield is 7.5 g. Calculate the percentage yield. [2]
Section C: Empirical and Molecular Formulae
6. A compound contains 24 g of carbon and 4 g of hydrogen. Determine the empirical formula. [3]
7. The empirical formula of a compound is CH₂. The $M_r$ is 56. Find the molecular formula. [3]
Total marks: 20
Mark Scheme
1. (a) Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g) [2] (b) CaCO₃(s) → CaO(s) + CO₂(g) [2]
2. H₂SO₄ = 98; Ca(OH)₂ = 74; (NH₄)₂SO₄ = 132 (1 each) [3]
3. 9 g H₂O: 9/18 = 0.5 mol; 4.0 g NaOH: 4.0/40 = 0.10 mol [2]
4. 8 g O₂ = 0.25 mol; ratio O₂:H₂O = 1:2 so H₂O = 0.5 mol; mass = 0.5 × 18 = 9 g [3]
5. (7.5/10)×100 = 75% [2]
6. C: 24/12=2; H: 4/1=4; ratio 1:2 → CH₂ [3]
7. Empirical mass = 14; 56/14=4; molecular formula C₄H₈ [3]