Question

how much oxygen is require to complete combustion of 8 gm of CH4

Answer 1

A combustion reaction of a hydro-carbon is governed by the reaction:
CxHy + (x + y/4) O2 = xCO2 + y/2 H2O
Here x=1 and y=4, so the reaction turns out to be
CH4 + 2O2 = CO2 + 2H20
so for 1 mole Methane 2 moles of Oxygen is required, ie, 16 gms of Methane requires 64 gms of Oxygen. So for 8 gm Methane 32 gm of Oxygen is required.