Hands on Science Sexto: Activity 16 - Oxygen and Combustion

I saw this simple demonstration in an infantil class and thought that there had to be a way of using this effect (once the oxygen is used up, water rises up to occupy the reduced volume) to measure the amount of oxygen in the air.  After a quick scribble of the chemical equation on paper, the approximate volume change should be about 1/2 the amount of oxygen in the air.  When I did this experiment myself, I was getting about 28% oxygen in the atmosphere;  it is actually about 21% which is not bad considering how basic the experiment is.

This activity is connected to the Matter unit.  I have written the activity with the students repeating the experiment 3 times and using average values to improve their data.  They also need to use their volume changes to calculate a percentage and they may need calculators to do this.  I have broken the calculation down into smaller steps, if the students are capable, let them do it without the extra help.  There a loads of errors to look out for in this experiment so lots of critical thinking opportunities!
  
Link to activity (pdf):  Oxygen and Combustion Activity