How to calculate the percentage by weight of water.?

question from text book



Calculate the percentage by weight of water in the following crystal:



Bluestone, CuSO4 . 5H20



need working + explanation, i think im doing something wrongHow to calculate the percentage by weight of water.?
anhydrous molar mass = 159.61 g mol-1.

pentahydrate molar mass = 249.68 g mol-1.

difference = 90.07 g mol-1 = 5H2O.



(90.07 / 249.68) x 100 = 36.07% by mass water.How to calculate the percentage by weight of water.?
Thinking yourself: smashing idea