Showing that air contains about one-fifth oxygen

2 marks · Easy

In an experiment to find the percentage of oxygen in air using iron filings, the total air volume at the start is 180 cm³. At the end, the volume is 143 cm³. Calculate the percentage of oxygen in the air. Give your answer to the nearest whole number.

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21% (±1)

Also accept: 21

Common mistakes: 37 (reverse subtraction); incorrect calculation; forgetting to multiply by 100

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Marking points

M1 · 1 mark · Subtracts final volume from initial (180 - 143 = 37 cm³)

A1 · 1 mark · Calculates (37/180) × 100 = 20.56 ≈ 21% (follow-through)

Worked solution