Question

The crystalline salt $N{a}_{2}S{O}_4.x{H}_{2}O$ on heating loses 55.9% of its mass and becomes anhydrous. The formula of the crystalline salt is:

• A. $N{a}_{2}S{O}_4.5H_{2}O$
• B. $N{a}_{2}S{O}_4.7H_{2}O$
• C. $N{a}_{2}S{O}_4.2H_{2}O$
• D. $N{a}_{2}S{O}_4.10H_{2}O$

Answer 1

The correct option is D $N{a}_{2}S{O}_4.10H_{2}O$

Molecular mass of $N{a}_{2}S{O}_4.x{H}_{2}O=142+18x$
On heating, crystalline salt will lose water of crystallization and eventually lose its weight.
So, 55.9% is the mass % of water dehydrated from total amount i.e. $18x$
$55.9=\frac{18x}{142+18x}\times 100$
Solving for $x$ gives, $x=9.99\sim 10$