From Yahoo AnswersIt is the total atomic mass of an element or compound. For example the mass of oxygen is O2 = 16 x 2 = 32 grams per mole. For water (H2O) it is the total weight of hydrogen (1 x 2 = 2 grams) plus the weight of oxygen (16 grams) = 18 grams per mole. If you are given 36 grams of water divide this by the weight of 1 mole to find out how many moles you have. 36 / 18 = 2 moles. Since you... Full answer