Question

100 cc of CO was mixed with excess of oxygen and exploded .find the vol. of oxygen used and vol.of CO2 formed

Answer 1

2H2 + O2 = 2H2O

It means that all H2 reacts with half of O2

But you have another process here:

2CO + O2 = 2CO2

If you have ten ml of CO and ten ml of O2, the O2 is in excess.
Ten ml of CO reacts only with 5 ml of O2 with producing ten ml of CO2,so in the end you have 10 + 5 =15 ml of gases mixture.