A hydrated form of copper sulfate ( CuSO 4 ⋅ x H 2 O ) is heated to drive off all of the water. If there is initially 7.74 g of hydrated salt and there is 4.95 g of anhydrous CuSO 4 after heating, find the number of water molecules associated with each CuSO 4 formula unit.

5 water molecules

Explanation:

Decomposition of hydrated copper sulphate is given by:

Molar mass of =  159.6 g/mol

According to stoichiometry:

(159.6+18x) g of decomposes to give = 159.6g of

Thus 7.74 g of   decomposes to give= g

But it is given 7.74 g of a sample of hydrated salt  was found to contain 4.95 g of anhydrous salt.

Thus we can equate the two equations:

x=5

Thus there are 5 water molecules associated with each formula unit.