Question

Change in volume when 100ml PH3 decomposed to solid phosphorus and H2 gas is.....?

Answer 1

If you produce the same number of H2 molecules as the number of PH3 molecules you started with, then the volume will stay the same. If less, or more, then the volume will be less or more in exactly the same proportion. (e.g. if you have twice as many H2 molecules, the volume will be twice as large.)