This is always a tough lab for students, it depends as much on performing the procedure right as on performing the calculation right. And those are both so daunting, they affect your performance of the other. Borek:
has already pointed you to the ideal gas law, so start by writing the formula. The flask, no matter how much liquid there was, had nothing but vapor in it once it was entirely evaporated. How much vapor? You have two answers for that question. Pull it all together with the other data in the formula.