Suppose I put 1 kg of -10-degree ice in a 1 m^3 vacuum chamber. How much heat do I have to add to turn all of the ice into vapor at 100 degrees Celsius? This seems like a ridiculously simple problem, but I realized I've no idea how to solve it. First of all, ice sublimates in a vacuum, which complicates the problem a lot. Then there's the problem that when the water boils, the newly created gas has to do work against the existing pressure to expand. Any ideas?