2.17     explain how experiments involving the reactions of elements such as copper, iron and phosphorus with air can be used to determine the percentage by volume of oxygen in air

Copper, iron or phosphorus can be put into a glass tube attached to gas syringes as shown below.

100 cm3 of air is passed backwards and forwards through the tube as it is heated. The element oxidises as it reacts with the oxygen in the air.  When it has been allowed to cool, the final volume is measured. The expected volume is 79 cm3.  The gas remaining is mostly nitrogen.