3.6 Reacting masses from equations
5 exam questions worth 14 marks. Write your answer, open the mark scheme, then give yourself the marks you earned. Your answers are saved on this device.
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In a reacting mass calculation, what is the first step when you know the mass of a reactant?
Choose one answer.
Correct Not quite: the answer is Divide the mass by its Mr
Mark scheme
A
Model answer
A: first convert the mass of the reactant into moles by dividing by its Mr.
Calculate the mass of carbon dioxide produced when 50 g of calcium carbonate is heated.
CaCO3 → CaO + CO2 (M_\mathrm{r}: CaCO3 = 100, CO2 = 44)
Mark scheme
- moles CaCO3 = 50 ÷ 100 = 0.50 (1)
- 0.50 mol CO2 (1:1 ratio) (1)
- 0.50 × 44 = 22 g (1)
Model answer
0.50 mol of CaCO3 gives 0.50 mol of CO2, which is 0.50 × 44 = 22 g.
Iron is made from iron(III) oxide: Fe2O3 + 3CO → 2Fe + 3CO2
Calculate the mass of iron made from 16 g of iron(III) oxide. (A_\mathrm{r}: Fe = 56, O = 16)
Mark scheme
- M_\mathrm{r} Fe2O3 = 160 (1)
- moles Fe2O3 = 16 ÷ 160 = 0.10 (1)
- moles Fe = 0.20 (1:2 ratio) (1)
- mass Fe = 0.20 × 56 = 11.2 g (1)
Model answer
M_\mathrm{r} = 160, so 0.10 mol of Fe2O3 gives 0.20 mol of Fe, which is 0.20 × 56 = 11.2 g.
4.6 g of sodium reacts with 1.6 g of oxygen, O2, to form 6.2 g of sodium oxide, Na2O.
Use these masses to work out the balanced equation. (A_\mathrm{r}: Na = 23, O = 16)
Mark scheme
- moles: Na 0.20, O2 0.050, Na2O 0.10 (1)
- ratio 4 : 1 : 2 (1)
- 4Na + O2 → 2Na2O (1)
Model answer
Na = 4.6 ÷ 23 = 0.20, O2 = 1.6 ÷ 32 = 0.050, Na2O = 6.2 ÷ 62 = 0.10. Dividing by 0.050 gives 4 : 1 : 2, so 4Na + O2 → 2Na2O.
Hydrogen burns in oxygen: 2H2 + O2 → 2H2O
Calculate the mass of water formed from 1.0 g of hydrogen. (A_\mathrm{r}: H = 1, O = 16)
Mark scheme
- moles H2 = 1.0 ÷ 2 = 0.50 (1)
- moles H2O = 0.50 (1)
- mass = 0.50 × 18 = 9.0 g (1)
Model answer
0.50 mol of H2 gives 0.50 mol of H2O, which is 0.50 × 18 = 9.0 g.
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