True, the volume decreases by 33%
Work Step by Step
We have 2 CO + O2 --> 2 CO2 In this reaction, we go from 3 moles on the left to 2 moles on the right. The ideal gas law is PV = nRT we are told that the piston maintains constant pressure and constant temperature. Therefore, the only thing changing is the number of moles, n, from 3 to 2, which means the volume must decrease from 3 to 2. If it was originally 3 and it increases to 2, this is an decrease of 1, which is an decrease of 33%.