Oxygen Percentage in Air

IGCSE Edexcel Chemistry
2.10 Determining the percentage of oxygen in air
Key Concept: Oxygen can be removed by reacting it with a metal. The drop in gas volume gives the percentage of oxygen in air.

Section A: Practical Method

1. Describe a method for determining the percentage of oxygen in air using heated copper. [3]
2. Why does the gas volume decrease during the experiment? [2]
3. A 100 cm3 air sample is passed over hot copper. The final volume is 79 cm3. Calculate the percentage of oxygen. [2]
4. State two precautions to improve accuracy. [2]

Total marks: 9

Mark Scheme

1. Pass air back and forth over heated copper in a sealed apparatus; copper reacts with oxygen to form copper oxide; measure volume change [3]
2. Oxygen is removed as it reacts to form copper oxide [2]
3. Oxygen = 21 cm3, percentage = 21% [2]
4. Use excess copper; ensure airtight system; allow to cool before reading; repeat until volume constant [2]