Volume increases... But I still don't see how that is involved in the problem...

Work has been done by the system if volume increases.

Work= P*change V

For a), I assume an arrow was meant after COCl2(g), you therefore have more mol of gas on the products side than reactants side. Given P and T are constant, the ideal gas law tells us that V increases and hence work is done by the system.

Note that if V decreases, work is done by the surroundings on the system.

We are only taking into account moles of gas for this by the way - do not count (s) or (l) as they won't influence the pressure.

Edit: (l) instead of (g)