Is 400g of oxygen gas enough to burn 20 moles of methane completely?
No
The combustion of methane is
CH4+2O2→ CO2+2H2O
i.e., One mole of methane requires two moles of oxygen gas.
i.e., 1 mole = (2 × 32)g of O2
20 moles of CH4 = 20 × (2 × 32)g of O2
20 moles of CH4 = 1280g of O2
i.e., 20 moles of CH4 will require 1280g of O2 to burn completely
Now, since there is only 400g of O2 gas, some CH4 can-not be burnt and will be left over after the reaction.
So, in this reaction, O2 is the limiting reagent as it gets used up completely.
So, in this reaction, CH4 is the excessive reagent.