Gases in the Atmosphere
7 marks · HardThis question is about experiments to measure oxygen in air and the formation of carbon dioxide from metal carbonates.
Describe how an experiment using wet iron filings can be used to determine the percentage of oxygen in air. Include the main steps and how the data is used.
A student heats 1.50 g of copper(II) carbonate and collects 275 cm³ of gas at room temperature and pressure. (i) Name the gas collected. (ii) Write a balanced equation for the reaction. (iii) Explain why the solid left at the end of the experiment has a different colour to the original solid.
Name the gas collected.
Write a balanced equation for the reaction.
Explain why the solid left at the end of the experiment has a different colour to the original solid.
State and explain one precaution that should be taken when interpreting the percentage of oxygen obtained from the iron experiment.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
(a) Wet iron filings are placed in a flask connected to a gas syringe. The initial air volume is measured. The flask is left for several days to allow rusting. The final air volume is measured. The decrease in volume is due to the oxygen reacting with iron. Percentage oxygen = (initial volume - final volume)/initial volume × 100%.
(b(i)) Carbon dioxide
(b(ii)) CuCO3(s) -> CuO(s) + CO2(g)
(b(iii)) The original solid is green copper(II) carbonate; heating forms black copper(II) oxide, so the colour changes from green to black.
(c) Ensure the iron is in excess/experiment is left long enough so all oxygen reacts; otherwise, the percentage obtained will be too low.
Common mistakes: Not mentioning water is needed for rusting; Confusing colour changes of copper(II) compounds; Equation for combustion instead of decomposition
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · Describes setting up wet iron filings in a closed container with known air volume and gas syringe.
M2 (a) · 1 mark · Mentions leaving the experiment long enough for rusting/iron to react with oxygen.
A1 (a) · 1 mark · States how to calculate percentage oxygen from the change in volume.
B1 (b(i)) · 1 mark · Identifies the gas collected as carbon dioxide.
B2 (b(ii)) · 1 mark · Writes correct balanced equation: CuCO3(s) -> CuO(s) + CO2(g)
B3 (b(iii)) · 1 mark · Explains colour change due to formation of black copper(II) oxide from green copper(II) carbonate.
B1 (c) · 1 mark · Names a valid precaution (e.g., iron in excess or enough time for reaction) with explanation.
Worked solution
a) Wet iron filings are placed inside a conical flask connected to a gas syringe. Measure the total air volume. Leave for several days so the iron reacts (rusts) with oxygen (needs water present). Measure the final volume of air when no further change is seen. The decrease in gas volume equals the volume of oxygen used up. To calculate the percentage of oxygen: (initial volume - final volume)/initial volume × 100%.
b) (i) The gas collected is carbon dioxide. (ii) CuCO3(s) → CuO(s) + CO2(g) (iii) The original solid, copper(II) carbonate, is green. When heated, it forms black copper(II) oxide, so the colour changes from green to black.
c) One precaution is to ensure the iron is in excess, or the experiment is left running long enough for all the oxygen to react with the iron. If not, the percentage of oxygen measured will be lower than the true value.