Question

Hydrated sulphate of a divalent metal of atomic weight 65.4 loses 43.85% of its weight on dehydration. Find the number of molecules of water of crystallisation in the formula of hydrated salt.

• A. $MS{O}_4\cdot 16H_{2}O$
• B. $MS{O}_4\cdot 5H_{2}O$
• C. $MS{O}_4\cdot 10H_{2}O$
• D. $MS{O}_4\cdot 12H_{2}O$

Answer 1

Answer: (A)

$MS{O}_4\cdot 16H_{2}O$

The formula of the salt will be $MS{O}_4.n{H}_{2}O$

Molar mass of the salt will be = 65.4+32+4×16 + n×18 = 161.4+18 n

On dehyration, all $H_{2}O$ molecules will evaporate and the formula of the salt becomes $MS{O}_4$ having molar mass 161.4

Therefore, we can say that

35.85% of (161.4+18n) = 161.4

or, 35.85100×(161.4+18n)=161.4

or, 161.4+18n =$\frac{161.4\times 100}{35.85}$

or, 161.4+18n = 450.20

or 18 n = 450.20−161.4 = 288.80or, n = $\frac{288.80}{18}$ = 16

Hence, the number of molecules of water of crystallisation = 16