Question

a student takes 4.597g of hydrated barium chloride and heats it in a crucible. The anhydrous material weighed 3.919g. What is the complete formula for this hydrated salt?

Answer 1

g H2O = 4.597 - 3.919 = 0.678
g BaCl2 = 3.919

Convert 0.678 g H2O moles. Moles = grams/molar mass
Convert 3.919 g BaCl2 to moles the same way.
Now fine the whole number ratio between the two. The easy way to do that is to divide the moles BaCl2 by itself (since you want 1.00 mole BaCl2) and this will give you exactly 1.00 mole. Then divide the moles water by the moles BaCl2 to find x in the formula, BaCl2.xH2O.