A chemist heats a sample of copper(II) sulfate and finds that 63.9% of the mass is copper(II) sulfate and the other 36.1% by mass is the water. What is the formula of the hydrate?

Answer 1

Take a 100 g sample to give you
63.9 g CuSO4 and
36.1 g H2O

Convert to mols.
mols CuSO4 = grams/molar mass = ?
mols H2O = grams/molar mass = ?
Now find the ratio of the two with the smaller number no less than 1.00. The easy way to do that is to divide the smaller number by itself (thereby getting 1.00). Then divide the other number by the same small number. Round to whole numbers. That will give you (CuSO4)1.00.xH2O where x is the other whole number.

Answer 2

x is 5