2.0 g of hydrated copper sulphate gave on heating,1.2786 g of anhydrous salt.calculate the number of molecules of water of crystallisation per molecule of hydrated salt.

Heating the will lead to the evaporation of the water of crystallization. The mass of water evaporated can be obtained by subtracting the mass of the residue obtained

From the question above, 

mass of the copper sulfate crystal = 2 g
Mass of residue = 1.2786 g
Mass of water evaporated  = 2g−1.2786g = 0.72 g

Lets assume that the number of molecules of water - x.

The number of molecules of water, x, is obtained by using the formula:
Mass of evaporated water⁄Mass of crystal = Molar Mass of evaporated water⁄Molar Mass of crystal 
Molar Mass of evaporated water = 18x
Molar Mass of crystal = (18x+160)
18x⁄(18x+160 )= 0.72⁄2
2(18x) = 0.72(18x + 160)
36x = 12.96x + 115.2
36x-12.96x = 115.2
23.04x = 115.2
x = 5
Therefore, the number of molecules of water, x, in the crystal is 5.

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