b0052 gas stoichiometry

Prelim question about combustion of methane :

Answer : A

There are two reactions involved in this experiment.

reaction 1 is combustion of methane.
reaction 2 is acid base reaction.
A lot of JC student forget that carbon dioxide is 
an acidic gas.
So they do not use reaction 2 in the calculation.

This is the calculation table :

There are only 3 gases in reaction 1 and reaction 2 :
Gas 1 = methane
Gas 2 = oxygen
Gas 3 =  carbon dioxide
So the table calculate the volumes of these 3 gases only.

potassium hydroxide,
and potassium carbonate
are not involved in the calculation because these substances are not gases.

Methane is the limiting reagent for reaction 1.
Oxygen is the excess reagent for reaction 1.
After reaction 1 there are 50 cm3 of oxygen and 50 cm3 of  carbon dioxide.

All carbon dioxide produced are removed after reaction 2.
so only 50 cm3 of oxygen left.
The answer is A.