An 82.0 g sample of hydrated magnesium sulfate (MgSO4) is heated to constant mass of 40.1 grams. What is the formula for the hydrate?

MgSO4.7H2O

Explanation:

The heating removes all the water (H2O) in the crystal.

The mass of the water removed = 82-40.1 = 41.9 grams.

The formula of the hydrated salt = MgSO4. xH2O

The ratio in of the masses of MgSO4 : H2O = 40.1 : 41.9

Dividing these by the molar masses we get

40.1 / 120.4 to 41.9 / 18.016

= 0.33 : 2.33

= 1 : 7.

So there are 7 water molecules n the hydrate.