Question

$10g$ of a crystalline metal sulphate salt when heated generates approximately $6.4g$ of an anhydrous salt of the same metal. The molecular weight of the anhydrous salt is $160g$. The number of water molecules present in the crystal is

• A. $1$
• B. $2$
• C. $3$
• D. $5$

Answer 1

The correct option is D $5$

Consider that the salt contains $\times$ molecules of water.
Molecular weight of anhydrous salt $=160g$
so molecular weight of hydrated salt will be $=160+18xg$
Then, no. of moles of water present in 10x gm of hydrated salt $=\frac{10}{160+18x}\times x$
and weight of water present in 10 gm of hydrated salt $=\frac{10x}{160+18x}\times 18$
$\underset{10g}{Hydrated}salt\to \underset{604g}{Anhydrous}salt+\underset{3.6g}{Water}$
$\frac{180x}{160+18x}=3.6$
$180x=576+64.8x$
$x=5$