Showing that air contains about one-fifth oxygen

3 marks · Normal

Below are two possible sources of error in an experiment using iron filings to measure the percentage of oxygen in air:

- The experiment is stopped after only one day, before the gas syringe reading stops changing.

- Only a small amount of iron filings is added, not in excess.

For each case, explain why the result might be inaccurate and whether it would make the calculated percentage of oxygen in air higher or lower than the correct value.

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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Must show: Each reason links error to percent oxygen being too low

Common mistakes: Confusing which error makes the result higher or lower

Show mark scheme and worked solution

Marking points

M1 · 1 mark · Explains stopping early: not all oxygen has reacted, so less volume decrease observed.

A1 · 1 mark · States stopping early causes percentage of oxygen result to be lower than expected.

M2 · 1 mark · Explains too little iron: not all oxygen can react, so decrease in volume is less than it should be, so percentage comes out too low.

Worked solution

If the experiment is stopped too soon, not all oxygen will have reacted, so the volume decrease will be smaller and the calculated percentage of oxygen will be too low.

If there is not enough iron, not all the oxygen will have reacted, so again, the measured percentage of oxygen will be lower than the true value.