Question: how to calculate percentage by mass of water of crystallization?
Answer: The % water of crystallisation in the crystals is 2.25 x 100 / 6.25 = 36% [ Ar's Cu=64, S=32, O=16, H=1 ] The mass ratio of CuSO4 : H2O is 4.00 : 2.25. To convert from mass ratio to mole ratio, you divide by the molecular mass of each 'species'