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

Using copper:

Set up the apparatus with 100cm3 of air in one of the gas syringes.

Heat the copper at one end of the silica tube using a blue bunsen flame.

Pass the air backwards and forwards over the copper

As the volume of gas in the syringe decreases, move the Bunsen flame along the tube so that it is always heating fresh copper.

Stop heating when the volume of gas in the syringe stops decreasing.

The copper is reacting with the oxygen in the air to form black copper oxide.
The final volume of the air left in the syringe will be approximately 79cm3, this shows that 21% of it has reacted, so there is 21% of oxygen in the air.