13.4 g of a sample of unstable hydrated salt Na₂SO₄. XH₂O was found to contain 6.3 g of H₂O. The number of  molecules of water of crystallisation is

Weight of salt = 13.4g

Weight of H₂O = 6.3g

Weight of anhydrous salt = 13.4 - 6.3 = 7.1g

Moles of anhydrous salt = $\frac{7.1}{842}=0.05$

Moles of H₂O = $\frac{6.3}{18}=0.35 \mathrm{mol}$

0.05 mol of anhydrous salt $\Rightarrow 0.35 \mathrm{mol} \mathrm{of} {\mathrm{H}}_2\mathrm{O}$

1 mole of anhydrous salt $\Rightarrow \frac{0.35}{0.05}=7 \mathrm{mol}$