Question

$13.4g$ of a sample of unstable hydrated salt, $N{a}_{2}S{O}_4.x{H}_{2}O$ was found to contain $6.3g\text{of}{H}_{2}O$. The number of molecules of water of crystallisation is:

• A. $5$
• B. $7$
• C. $2$
• D. $10$

Answer 1

The correct option is B $7$

Weight of salt $=13.4g$
Weight of $H_{2}0$ $=6.3g$
Weight of anhydrous salt $=13.4-6.3=7.1g$
Moles of anhydrous salt $=\frac{7.1}{842}=0.05$
Moles of $H_{2}O=\frac{6.3}{18}=0.35mol$
$0.05$ mol of anhydrous salt gives 0.35 mol of $H_{2}O$.
$1mol$ of anhydrous salt will give $\frac{0.35}{0.05}=7mol$ of water.