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]

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 = 136\over 79 × 100 = 172.15 g

21% of CaSO₄.xH₂O = 21\over 100 × 172.15

= 36.15 g

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

Number of molecules (x) = 36.15\over 18

= 2 molecules