How to calculate the results of the percentage of oxygen in the air.
(i) VOLUME OF AIR AT START = 100cm3
(ii) VOLUME OF AIR AT END = 80 cm3
(iii) THEREFORE VOLUME OF OXYGEN REMOVED = 20 cm3
iv) THEREFORE % OXYGEN = 20/100 x 100 = 20%
The 80cm3 of gases left in the syringes are called the RESIDUAL GASES.