Oxygen in Air Science Project Idea

How much of the air is oxygen?

Can you suggest a method by which this may be found out?

Air Science Experiment

On a metal disk on a flat piece of cork, place a bit of yellow phosphorus. Place the cork on water and invert over it a glass cylinder. Examine after two days. Result? Conclusion?

Quantitative experiments have shown that air has the following composition :
Oxygen, 20+ per cent, Nitrogen, including several inert gases, 79+ per cent, Carbon dioxide, .03 to .04 of 1 per cent.