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Question
A hydrate of calcium sulphate CaSO4.xH2O contains 21% water of crystallization. Find the value of x.
Solution
Molecular mass Ca + S + 4[O] + x[2H + O]
= 40 + 32 + 4(16) + x[(2 × 1) + 16]
= [136 + 18x] gram
% of water in molecule = `21/100`[136 + 18x]
= 18x
∴ [136 + 18x] × 7 = 600x
(136 × 7) + 126x = 600x
∴ 952 = (600 − 126)x
∴ 952 = 474x
∴ `952/474` = x
∴ x = 2 molecules
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