I did this a bunch of times and did not understand what to do. Hopefully someone can shed some light. Our prof gave us hints, but they aren't really helping. A mixture of KCr(SO3)2 and its hydrate KCr(SO4)2.12H2O has a mass of 1.6336 g. After heating (driving off water), the mass is 1.4659 g. What is the weight % of KCr(SO4)2.12H2O in the original mixture? Molar mass of KCr(SO4)2 is 283.22 g/mol Molar mass of KCr(SO4)2.12H2O is 499.40 g/mol. -------------------- This is what I tried: I figured the mass of water was 0.1677g, and therefore the moles were 0.0009306 mol. Then, the mols of KCr(SO4)2.12H2O is 0.0007755. The mass was that x 499.40, which gave me 0.38728 g. And then I was lost. I don't know how to utlize the molar mass of KCr(SO4)2 in this question, either. Thanks!