Edexcel IGCSE Chemistry · Spec 2.14
Percentage of Oxygen in Air
The experiment used to find the percentage of oxygen in air.
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Percentage of Oxygen in Air
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Explained
Removing the oxygen and measuring what is left
You cannot measure the oxygen in air directly, so this experiment does the next best thing. It takes the oxygen out and measures how much the volume of gas falls. Whatever is missing was the oxygen.
The gas syringe method
Two gas syringes are joined by a hard glass tube containing copper turnings. A measured volume of air, usually 100 cm cubed, is put in one syringe and the other is empty.
Heat the copper strongly and push the air backwards and forwards over it. The copper reacts with the oxygen to form black copper oxide, so the oxygen is removed from the gas.
Let it cool to room temperature before reading the volume, otherwise you are measuring hot, expanded gas. Repeat the heating and passing until the volume stops changing, which is what shows all the oxygen has reacted.
The final volume is around 79 cm cubed, so about 21 cm cubed has gone, which is the oxygen.
Why copper
Copper reacts with oxygen when heated but not with nitrogen, so it removes exactly one component and leaves the rest. It also stays solid, so it does not add any gas of its own.
Using excess copper matters: if the copper runs out before the oxygen does, the volume stops falling for the wrong reason and your answer comes out too low.
The calculation
Percentage of oxygen equals the decrease in volume, divided by the starting volume, multiplied by 100.
Starting at 100 cm cubed and finishing at 79 gives a decrease of 21, so 21 divided by 100 times 100 is 21 per cent.
Divide by the starting volume, not the final one. That is the single most common arithmetic error here.
What the mark scheme accepts and rejects
An Edexcel International GCSE Chemistry mark scheme takes a version of this experiment where the gas column fell from 93 mm to 77 mm. It awards one mark for the difference, 16 mm, and a second for 16 divided by 93, multiplied by 100, giving 17 per cent. Its note says that the second mark subsumes the first, so a correct final answer earns both even with no working shown.
Two things are worth taking from that. The measurement is divided by the original height, not the remaining one. And the answer was 17 per cent rather than 21.
That gap is the experiment, not a mistake in the mark scheme. A real result comes out below 21 per cent if not all the oxygen has reacted, and questions often ask you to explain a value that is too low. The reasons are that the copper was not heated for long enough, that the gas was not passed over it enough times, that there was not enough copper, or that the apparatus leaked.
A result that is too high is harder to explain, and the usual cause is reading the volume while the gas is still hot.
Simpler versions of the same idea
Burning a candle in a sealed jar over water, or leaving damp iron wool in an inverted test tube over water, both work on the same principle: something removes the oxygen and the water rises to fill the space.
Both give answers below 21 per cent, and for a reason worth knowing. A candle goes out before all the oxygen is used, because a flame needs a certain concentration to keep burning. Iron wool rusting is slow but does eventually use nearly all of it, which is why the rusting version gives a better result if you leave it a week.
Spec 2.14
What you need to know
- Describe an experiment to find the percentage of oxygen in air
- Explain the results
- Calculate the percentage of oxygen
Active recall
Quick check
Answer each question before opening the answer.
How can you show that air is about 20% oxygen?
Pass a fixed volume of air over heated copper (or burn something) so the oxygen is used up, and measure the decrease in volume.
What result would you expect?
The volume of air decreases by about one fifth (20%).
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