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

The elements mentioned above all react with air (copper, iron and phosphorus). You could set up the following experiment:

• Set up the apparatus shown in the video :)
• You must have a know volume of air, as shown
• Heat the copper bits evenly, and keep pressing down one syringe and then the other, so the air passes through the copper
• The oxygen will react with the copper to form copper oxide and will be 'taken away' from the air
• Keep repeating step 3 until the volume of air stops decreasing
• Turn off the bunsen burner (or put it aside) 
• Leave to cool for 5 minutes
• Measure final reading. The volume of air should have gone down. 
• Calculate the percentage of decrease in volume of air. This is the percentage of oxygen there was.