J.R. S. answered 02/22/18
Ph.D. University Professor with 10+ years Tutoring Experience
The volume is proportional to the number of moles of gas present. Notice that there are 2 moles on the left (reactant side) but 3 moles on the right (product side). This represents a 50% increase in the number of moles of gas present, thus the volume increases by this amount, if the pressure and temperature remain constant. Looking at PV = nRT and solving for n (moles of gas), you can see this.
n = PV/RT and if the pressure and temperature remain constant (and R is also a constant), then the volume will increase in proportion to the number of moles (n).