How much volume of oxygen will be required for complete combustion of 40 ml of acetylene and how much volume of CO2 and H2O will be formed?
The volume can change in the course of the reaction. For example, if you add $\ce{Zn}$ to $\ce{HCl}$ the volume of the system changes. In the reaction $\ce{2H2 + O2 -&gt; 2H2O}$ three volumes of gases turn into 1 (or even zero, if you allow water to condensate).

While mass remains constant, volume can change in chemical reactions.

Behavior of gases (under low pressure) is described by ideal gas theory. In gas a molecule is a small particle that has a large free space around it. The amount of free space is independent from of the molecular mass. lightest molecule $\ce{H2}$ occupies as much space as a heavy gas $\ce{SF6}$. As a result, density of $c\ce{H2}$ is much smaller than density of air (this is how helium baloons fly) which in turn is smaller than density of $\ce{SF6}$.

If two molecules form a dimer the density of the gas doubles, and the volume decreases by a factor of two (if the pressure is kept constant. 

From 12 carbons and 24 hydrogens you can make 6 molecules of ethane ($\ce{C2H4}$) or 4 molecules of cyclopropane ($\ce{C3H8}$) or three molecules of cyclobutane ($\ce{C4H8}$). The amount of atoms is the same, the weight is conserved, but the volume and density are not.