Question

A crystalline hydrated salt on being rendered anhydrous loses 45.6% of its weight.
The percentage composition of the anhydrous salt is:
$Al=10.5\%,K=15.1\%,S=24.8\%$ and oxygen $=49.6\%$
[Molar mass of $Al=27,K=39,S=32$].

What is the empirical formula of the hydrated salt?

• A. $KAl{S}_2{O}_8.10H_{2}O$
• B. $KAl{S}_2{O}_8.16H_{2}O$
• C. $KAl{S}_2{O}_8.8H_{2}O$
• D. $KAl{S}_2{O}_8.12H_{2}O$

Answer 1

The correct option is D $KAl{S}_2{O}_8.12H_{2}O$

Assume molecular formula of hydrated salt = $KAl{S}_2{O}_8.x{H}_{2}O$

Then, $\frac{18x}{39+27+64+128+18x}\times 100=45.6$
$\Rightarrow x=12$.