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

states-of-matter
moderate
gaseous-state
find-percentage-composition
unsolved
pp
#1

I am studying Gaseous State.

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

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

:Volume of CO2= 68.5mL-38.5mL . How come? Also please clarify why nitrogen does not undergo combustion.

Please help.

#2

Nitrogen is exceptionally stable cuz of 3 bonds . It doesn't reacts easily . It reacts with oxygen above 2000 ℃ or in electric spark .
KOH absorbs CO_2 so volume change is CO_2 loss.