Topic 3.2.2: Reacting Masses
Past Paper Questions - Conservation & Proportional Logic
Examination Instructions
- •Answer all questions. Show all working clearly.
- •Use the Law of Conservation of Mass: Total mass of reactants = Total mass of products.
- •Pay attention to the molar ratios from balanced equations.
- •Give answers to appropriate significant figures (usually 2 or 3 s.f.).
Section A: Conservation of Mass (25 marks)
Foundation - Understanding that mass is conserved in reactions
Question 1
[2 marks]When 4 g of hydrogen reacts completely with 32 g of oxygen, water is formed.
Calculate the mass of water produced.
Question 2
[2 marks]Magnesium burns in oxygen to form magnesium oxide:
2Mg + O₂ → 2MgO
If 6 g of magnesium reacts with 4 g of oxygen, what mass of magnesium oxide is formed?
Question 3
[3 marks]A student heats 10.0 g of copper carbonate (CuCO₃). The reaction is:
CuCO₃ → CuO + CO₂
After heating, 6.4 g of copper oxide (CuO) remains in the test tube.
Calculate the mass of carbon dioxide gas that escaped.
Question 4
[3 marks]Limestone (calcium carbonate) thermally decomposes:
CaCO₃ → CaO + CO₂
A sample of 50 g of calcium carbonate is heated completely.
The mass of calcium oxide (quicklime) produced is 28 g.
What mass of carbon dioxide was released?
Question 5
[4 marks]Iron reacts with sulfur to form iron sulfide:
Fe + S → FeS
In an experiment, 2.8 g of iron reacts with 1.6 g of sulfur.
(a) Calculate the mass of iron sulfide produced. [2]
(b) Explain why the mass of the container stays the same during this reaction. [2]
Section B: Scaling Reacting Masses (15 marks)
Using proportions from balanced equations
Question 6
[3 marks]The reaction between hydrogen and oxygen is:
2H₂ + O₂ → 2H₂O
When the reaction uses 4 g of hydrogen and 32 g of oxygen, it produces 36 g of water.
Calculate the mass of water produced from 1 g of hydrogen (with excess oxygen).
Question 7
[3 marks]Methane burns in oxygen:
CH₄ + 2O₂ → CO₂ + 2H₂O
When 16 g of methane burns completely, 44 g of carbon dioxide is produced.
What mass of carbon dioxide is produced when 8 g of methane burns?
Question 8
[3 marks]Zinc reacts with hydrochloric acid:
Zn + 2HCl → ZnCl₂ + H₂
13 g of zinc produces 27.2 g of zinc chloride.
Calculate the mass of zinc chloride produced from 6.5 g of zinc.
Question 9
[3 marks]Carbon reacts with oxygen:
C + O₂ → CO₂
3 g of carbon produces 11 g of carbon dioxide.
What mass of carbon dioxide is produced from 12 g of carbon?
Question 10
[3 marks]Nitrogen and hydrogen react to form ammonia:
N₂ + 3H₂ → 2NH₃
28 g of nitrogen reacts with 6 g of hydrogen to produce 34 g of ammonia.
Calculate the mass of ammonia produced from 14 g of nitrogen (with excess hydrogen).
Section C: Industrial Scale Calculations (8 marks)
Real-world applications with Mr ratios
Question 11
[4 marks]Iron is extracted from iron(III) oxide in a blast furnace:
Fe₂O₃ + 3CO → 2Fe + 3CO₂
Relative formula masses: Fe₂O₃ = 160, Fe = 56
A blast furnace processes 800 tonnes of Fe₂O₃. Calculate the maximum mass of iron that can be produced.
Question 12
[4 marks]Calcium carbonate decomposes when heated:
CaCO₃ → CaO + CO₂
Relative formula masses: CaCO₃ = 100, CaO = 56, CO₂ = 44
A cement factory heats 5000 kg of calcium carbonate. Calculate:
(a) The mass of calcium oxide produced. [3]
(b) The mass of carbon dioxide released. [1]
Question 13
[4 marks]Aluminium is extracted by electrolysis of aluminium oxide:
2Al₂O₃ → 4Al + 3O₂
Relative formula masses: Al₂O₃ = 102, Al = 27
An aluminium smelter uses 2040 tonnes of Al₂O₃ per day. Calculate the mass of aluminium produced daily.
Question 14
[3 marks]Ammonia is manufactured by the Haber process:
N₂ + 3H₂ → 2NH₃
Relative formula masses: N₂ = 28, NH₃ = 17
A factory uses 140 kg of nitrogen. What is the maximum mass of ammonia produced?
Question 15
[5 marks]Copper is extracted from copper(II) sulfide ore:
2Cu₂S + 3O₂ → 2Cu₂O + 2SO₂
Cu₂O + C → 2Cu + CO
Relative formula masses: Cu₂S = 160, Cu = 64
A mine processes 3200 tonnes of Cu₂S ore daily.
(a) Calculate the mass of copper that could theoretically be extracted. [4]
(b) Suggest why the actual mass obtained might be lower. [1]
Key Principles for Reacting Mass Calculations
1. Conservation of Mass
Total mass of reactants = Total mass of products. Nothing is created or destroyed.
2. Molar Ratios
Use coefficients from balanced equations to find Mr ratios between substances.
3. Scale Factors
Divide actual mass by Mr to find the scaling factor, then multiply to find product mass.
4. Show Your Working
Always write down ratios, scale factors, and units. Partial marks are available!
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