Gases in the Atmosphere
5 marks · EasyA student uses an experiment involving heated copper to determine the percentage of oxygen in a sample of air. She then compares the combustion of magnesium in air and in pure oxygen.
The apparatus originally contains 80 cm³ of air. After the reaction with copper is complete and the apparatus has cooled, the remaining gas volume is 63 cm³.
Calculate the volume of oxygen removed from the air.
Calculate the percentage of oxygen in the air sample. Show your working.
Describe why it is important to use excess copper in this experiment.
Magnesium burns in air to produce a white powder. Explain how the combustion of magnesium differs when it burns in pure oxygen instead of in air.
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.
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(a(i)) 17 cm³ cm^3
(a(ii)) 21%
(b) To ensure all the oxygen in the air reacts, giving an accurate measurement; if not enough copper is used, not all oxygen will react and the percentage will be too low.
(c) Magnesium burns more brightly and rapidly in pure oxygen than in air, producing more heat and a more intense white flame. This is because pure oxygen allows more complete and rapid combustion than air, which contains only about 21% oxygen.
Common mistakes: Not subtracting volumes correctly; Forgetting to use initial and final readings; Generic explanations not specific to copper or magnesium
Show mark scheme and worked solution
Marking points
M1 (a(i)) · 1 mark · Correctly subtracts final volume from initial volume to find volume of oxygen removed.
A1 (a(ii)) · 1 mark · Accurately calculates percentage of oxygen using correct formula (volume removed/original volume × 100).
B1 (b) · 1 mark · Gives correct reason for using excess copper (to ensure all oxygen reacts/accurate result).
M2 (c) · 1 mark · States that magnesium burns more brightly (or similar) in pure oxygen than air or equivalent description.
A2 (c) · 1 mark · Explains this is due to higher oxygen concentration in pure oxygen than in air/allows more complete combustion.
Worked solution
a(i) The volume of oxygen removed = initial volume - final volume = 80 cm³ - 63 cm³ = 17 cm³.
a(ii) Percentage of oxygen in air = (volume removed ÷ initial volume) × 100
So the percentage of oxygen is about 21%.
b) Excess copper is used so that all the oxygen can react. If copper is not in excess, some oxygen would remain unreacted, giving a value that is too low.
c) Magnesium burns more brightly and rapidly in pure oxygen, producing a more intense white flame than in air. This is because air only contains about 21% oxygen, whereas pure oxygen allows more complete combustion, increasing the heat and light produced.