What is the cause of decrease in volume of gaseous mixture in the following problem? [closed]

I am studying Gaseous State.

The problem says: A mixture of gas contains $$\ce{N2}$$ and $$\ce{C2H2}$$. $$\pu{20 \\mL}$$ of this mixture is added to $$\pu{70\\mL}$$ of $$\ce{O2}$$ and combustion is allowed to take place over mercury. After cooling the volume of gas was found to be $$\pu{68.5\\mL}$$. When the resultant gas mixture is passed through a $$\ce{KOH}$$ solution, the volume of residual gas is $$\pu{38.5\\mL}$$. Then the percentage composition of $$\ce{N2}$$ is:

In the solution to the question there is this following step which I am not getting:

:Volume of $$\ce{CO2} = \pu{68.5\\mL} - \pu{38.5\\mL}$$. How come? Also please clarify why nitrogen does not undergo combustion.

closed as off-topic by Mithoron, Jon Custer, Tyberius, Mathew Mahindaratne, airhuffJul 13 at 4:55

This question appears to be off-topic. The users who voted to close gave this specific reason:

If this question can be reworded to fit the rules in the help center, please edit the question.

• Wait until you'll be studying Chemistry, and all this will start to make sense. There is no shortcut. – Ivan Neretin Jul 10 at 8:38
• But I need a bit help. – Shobhit Singhal Jul 10 at 8:40
• Chemistry is more than just a bit. It is a vast body of knowledge. There are whole textbooks on it, some of them thick enough to kill a man. – Ivan Neretin Jul 10 at 8:55
• For starters, nitrogen is pretty unreactive... It's chemistry alright, but it's also general knowledge. Never heard of aircraft tires filled with nitrogen? – William R. Ebenezer Jul 10 at 8:57
• Could please come up with a solution to the problem. I need it. – Shobhit Singhal Jul 10 at 9:56

I think the fact you are missing is that after combustion, water vapors will be given away and the volume lost during combustion will be equal to the amount of water lost. $$\ce{KOH}$$ will absorb $$\ce{CO2}$$ and the volume absorbed by $$\ce{KOH}$$ will be equal to volume of $$\ce{CO2}$$ in the solution. Thus, you will have two equations and you will be able to proceed with the solution.