Hydrated calcium sulphate [CaSO₄.xH₂O] contains 21% of water of crystallization. Calculate the number of molecules of water of crystallization i.e. 'X' in the hydrated compound.

[Ca = 40, S = 32, O = 16, H = 1]

Given,

CaSO₄.xH₂O contains 21% of water of crystallization, so % of CaSO₄ = 100 - 21 = 79%

Molecular weight of anhydrous calcium sulphate CaSO₄ = 40 + 32 + 4(16) = 40 + 32 + 64 = 136 g

i.e. 79% of CaSO₄.xH₂O = 136 g

∴ CaSO₄.xH₂O = $\dfrac{136}{79}$ x 100 = 172.15 g

21% of CaSO₄.xH₂O = $\dfrac{21}{100}$ x 172.15 = 36.15 g

Molecular weight of water H₂O = 2(1) + 16 = 18

Number of molecules (x) = $\dfrac{36.15}{18}$ = 2 molecules

∴ CaSO₄.xH₂O = CaSO₄.2H₂O