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. Only 12.8 g of copper was actually obtained. Calculate the percentage yield of copper. [2] Section F: Gas Volumes (Molar Gas Volume) 14. Calculate the volume occupied by 0.25 mol of carbon dioxide gas at room temperature and pressure (r.t.p.), where 1 mole of gas occupies 24 dm³ at r.t.p. [1] 15. Calcium carbonate reacts with excess dilute hydrochloric acid: CaCO ₃ + 2HCl → CaCl ₂ + H ₂ O + CO ₂ Calculate the volume of CO ₂ gas produced (at r.t.p.) when 5.0 g of calcium carbonate reacts completely. [3]

16. 120 cm³ of hydrogen gas reacts completely with chlorine gas to form hydrogen chloride gas: H ₂ + Cl ₂ → 2HCl Calculate the volume of hydrogen chloride gas produced (all volumes measured at the same temperature and pressure). [2] Section G: Concentration & Titration Calculations 17. Calculate the number of moles of solute in 250 cm³ of 0.20 mol/dm³ sodium hydroxide solution. [1] 18. 25.0 cm³ of 0.10 mol/dm³ sodium hydroxide solution exactly neutralises 20.0 cm³ of hydrochloric acid. NaOH + HCl → NaCl + H ₂ O Calculate the concentration of the hydrochloric acid, in mol/dm³. [3] 19. In a titration, 22.5 cm³ of 0.15 mol/dm³ sulfuric acid exactly neutralises 25.0 cm³ of sodium hydroxide solution. H ₂ SO ₄ + 2NaOH → Na ₂ SO ₄ + 2H ₂ O Calculate the concentration of the sodium hydroxide solution, in mol/dm³. [3] Section H: Water of Crystallisation 20. A hydrated salt has the formula CuSO ₄ ·xH ₂ O. When 5.0 g of this hydrated salt is heated to constant mass, 3.2 g of anhydrous copper(II) sulfate remains. Determine the value of x. [Mr: CuSO ₄ = 160, H ₂ O = 18] [3] Section I: Mixed/Applied Questions 21. A student wants to prepare 100 cm³ of 0.50 mol/dm³ copper(II) sulfate solution from solid copper(II) sulfate crystals, CuSO ₄ ·5H ₂ O. [Mr: CuSO ₄ ·5H ₂ O = 250] (a) Calculate the number of moles of CuSO ₄ ·5H ₂ O needed. [1] (b) Calculate the mass of CuSO ₄ ·5H ₂ O required. [1] (c) Describe briefly how the student would prepare this solution using the calculated mass. [2] 22. Ethanol burns completely in oxygen: C ₂ H ₅ OH + 3O ₂ → 2CO ₂ + 3H ₂ O Calculate the volume of oxygen gas (at r.t.p.) needed to completely burn 4.6 g of ethanol. [Mr: C ₂ H ₅ OH = 46] [3]

End of practice set — [Total: 60 marks] Tip: Attempt all questions first before checking answers. Show all working clearly, including balanced equations, mole ratios, and units at each step — this is where most marks are awarded in O-Level stoichiometry questions.