Question

$5$ g of crystalline salt, when rendered anhydrous, lost $1.89$ g of water. The formula weight of anhydrous salts is $160$. The number of molecules of water of crystallisation in the salt is:

• A. $1$
• B. $2$
• C. $3$
• D. $5$

Answer 1

The correct option is D $5$

Given, $1.89$ g of water is present in $5$ g of hydrated salt.

Therefore, weight of anhydrous salt is $(5-1.89)$ g $=3.11$ g.

The amount of water present with $160$ g of anhydrous salt will be $1.89\times \frac{160}{3.11}=97.23$ g.

Mass of one mole of water molecule is $18$ g.

This means that 1 mole of crystalline salt has $97.23g$ of water.

$\therefore$ $97.23$ g will correspond to $\frac{97.23}{18}=5.4\cong 5$ moles of water.

Hence, option D is correct.