Magnesium carbonate (MgCO3) is a hydrate. If you start with 15.67 g of the hydrate and after heating you get 7.58 g, what is the formula of the hydrate? I'm having a lot of trouble on this one.

Answer 1

how many grams of water evaporated?
15.67 - 7.58 = 8.09 gramsH2O
how many mols of water is that
mol of water = 2 + 16 = 18 grams
so this is just about 8.09 / 18 = 0.45 mol of water
NOW how many mols of MgCO3 did you have
7.58 grams of MgCO3
mol = 24.3 + 12 + 3*16 = 84.3 grams
so you have 7.58/84.3 = 0.09 mol of MgCO3
what is the ratio of mols H2O to mols MgCO3 ?
.45 / .09 = 5 on the nose !
MgCO3 * 5H2O