Topic 3.3.6: Practical Maths
Paper 4 Style Questions
% Composition
% Purity
% Yield
EXAMINATION INSTRUCTIONS
- •Total Questions: 15 (5 per section)
- •Time Allowed: 45 minutes
- •Calculator: Required
- •Show ALL working - Method marks are awarded even if final answer is incorrect
- ⚠️Significant Figures: Give answers to 2-3 significant figures unless stated otherwise
SECTION A: Percentage Composition by Mass
Questions 1-5 [2 marks each]
Question 1
[2 marks]Calculate the percentage by mass of carbon in methane (CH4).
[Ar values: C = 12, H = 1]
Question 2
[2 marks]Calculate the percentage by mass of hydrogen in water (H2O).
[Ar values: H = 1, O = 16]
Question 3
[2 marks]Ammonia (NH3) is used to manufacture fertilizers. Calculate the percentage by mass of nitrogen in ammonia.
[Ar values: N = 14, H = 1]
Question 4
[3 marks]Calcium carbonate (CaCO3) is the main compound in limestone. Calculate the percentage by mass of oxygen in calcium carbonate.
[Ar values: Ca = 40, C = 12, O = 16]
Question 5
[3 marks]Magnesium sulfate (MgSO4) is used in bath salts. Calculate the percentage by mass of magnesium in magnesium sulfate.
[Ar values: Mg = 24, S = 32, O = 16]
SECTION B: Percentage Purity
Questions 6-10 [2-3 marks each]
Question 6
[2 marks]A 10 g sample of limestone contains 9 g of pure calcium carbonate (CaCO3). Calculate the percentage purity of the limestone.
Question 7
[3 marks]An iron ore sample has a total mass of 500 tonnes. It contains 25 tonnes of impurities. Calculate the percentage purity of the iron ore.
Question 8
[3 marks](a) A 50 g sample of malachite contains 38 g of copper carbonate. Calculate the percentage purity. [2]
(b) Suggest two reasons why the purity is less than 100%. [1]
Question 9
[2 marks]A chemical company produces 200 g of sodium chloride (NaCl) with a purity of 85%. Calculate the mass of pure sodium chloride produced.
Question 10
[3 marks]A sample of copper sulfate has a purity of 95%. If it contains 190 g of pure copper sulfate, calculate the total mass of the sample.
SECTION C: Percentage Yield
Questions 11-15 [2-4 marks each]
Question 11
[2 marks]In an industrial process, the theoretical yield of a product is 50 tonnes. The actual yield obtained is 40 tonnes. Calculate the percentage yield.
Question 12
[3 marks](a) A student calculated that 8.0 g of copper oxide should be produced. They actually obtained 6.8 g. Calculate the percentage yield. [2]
(b) Suggest one reason why the percentage yield is less than 100%. [1]
Question 13
[2 marks]The theoretical yield of ammonia in the Haber process is 120 tonnes. The percentage yield is 75%. Calculate the actual yield of ammonia produced.
Question 14
[4 marks]10 g of calcium carbonate (CaCO3) is heated to produce calcium oxide (CaO):
CaCO3 → CaO + CO2
The actual yield of calcium oxide obtained is 5.0 g. Calculate the percentage yield.
[Ar values: Ca = 40, C = 12, O = 16]
Question 15
[4 marks]In the Contact Process for making sulfuric acid, a factory obtains 184 tonnes of product with a percentage yield of 92%. Calculate the theoretical yield.
📊 EXAM FORMULA SHEET
% Composition
(Mass of element ÷ Mr) × 100
% Purity
(Mass of pure ÷ Total mass) × 100
% Yield
(Actual ÷ Theoretical) × 100
🎯 TOP EXAM TIPS:
- ✓ Always multiply by 100 (or you get a decimal, not a percentage)
- ✓ Show ALL working (method marks can save you even if final answer is wrong)
- ✓ Give answers to 2-3 significant figures
- ✓ For yield questions, calculate theoretical yield FIRST using stoichiometry
- ✓ Units matter: tonnes ÷ tonnes = no units, but g ÷ kg = wrong!
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