O-Level Pure Chemistry_ Mole Concept & Stoichiometry

O-Level Pure Chemistry: Mole Concept & Stoichiometry Prelim Practice Questions Section A: Basic Mole Calculations 1. Calculate the number of moles in: (a) 11.5 g of sodium (Na) [1] (b) 4.4 g of carbon dioxide, CO ₂ [1] (c) 3.01 × 10²³ molecules of oxygen gas, O ₂ [1] 2. Calculate the mass of: (a) 0.25 mol of calcium carbonate, CaCO ₃ [1] (b) 2.5 mol of water, H ₂ O [1] 3. Calculate the number of particles (atoms/molecules) in: (a) 0.5 mol of neon gas, Ne [1] (b) 0.1 mol of ammonia, NH ₃ [1] Section B: Empirical & Molecular Formula 4. A compound contains 40.0% carbon, 6.7% hydrogen and 53.3% oxygen by mass. Determine the empirical formula of this compound. Show your working. [3] 5. A hydrocarbon has the empirical formula CH ₂ . Its molar mass is 84 g/mol. Deduce the molecular formula of this hydrocarbon. [2] 6. 2.30 g of an oxide of nitrogen contains 0.70 g of nitrogen and the rest oxygen. Determine the empirical formula of this oxide. [3] Section C: Reacting Masses 7. Magnesium reacts with hydrochloric acid according to the equation: Mg + 2HCl → MgCl ₂ + H ₂ Calculate the mass of magnesium chloride formed when 4.8 g of magnesium reacts completely with excess hydrochloric acid. [3] 8. Calcium carbonate decomposes on heating: CaCO ₃ → CaO + CO ₂

Calculate the mass of calcium oxide formed when 25.0 g of calcium carbonate is completely decomposed. [3] 9. Iron reacts with oxygen to form iron(III) oxide: 4Fe + 3O ₂ → 2Fe ₂ O ₃ Calculate the mass of iron needed to produce 32.0 g of iron(III) oxide. [3] Section D: Limiting Reagent 10. 5.0 g of zinc is added to 100 cm³ of 0.5 mol/dm³ hydrochloric acid. Zn + 2HCl → ZnCl ₂ + H ₂ (a) Calculate the number of moles of zinc used. [1] (b) Calculate the number of moles of hydrochloric acid used. [1] (c) Determine the limiting reagent, showing your working. [2] (d) Calculate the maximum mass of zinc chloride that could be formed. [2] 11. 3.4 g of ammonia, NH ₃ , is reacted with 8.0 g of oxygen gas. 4NH ₃ + 5O ₂ → 4NO + 6H ₂ O Determine which reactant is in excess, and calculate the mass of NO formed. [4] Section E: Percentage Yield & Percentage Purity 12. A student reacts 10.0 g of impure calcium carbonate with excess hydrochloric acid. The mass of pure calcium carbonate that reacted was calculated to be 8.5 g. Calculate the percentage purity of the calcium carbonate sample. [2] 13. In an experiment, 20.0 g of copper(II) oxide is expected to produce 16.0 g of copper.