The crystalline slat ${\mathrm{Na}}_2{\mathrm{SO}}_4·{\mathrm{xH}}_2\mathrm{O}$ on heating loses 55.9 % of its weight. The formula of crystalline salt is ___?

As ${\mathrm{Na}}_2{\mathrm{SO}}_4·{\mathrm{xH}}_2\mathrm{O}$ on heating loses water, i.e., 55.9 % $\approx 56\%$of its weight.

$$\begin{gathered} \therefore \%\text{ of weight of }{\mathrm{Na}}_2{\mathrm{SO}}_4=44\% \\ \text{ Molecular weight of }{\mathrm{Na}}_2{\mathrm{SO}}_4=142 \mathrm{g} \\ \therefore \frac{{\mathrm{w}}_{{\mathrm{H}}_2\mathrm{O}}}{{\mathrm{w}}_{{\mathrm{Na}}_2{\mathrm{SO}}_4}}=\frac{56}{44}{\mathrm{orW}}_{{\mathrm{H}}_2\mathrm{O}}=\frac{56}{44}\times 142=180 \mathrm{g} \\ \text{ Thus, the formula of crystalline salt is }{\mathrm{Na}}_2{\mathrm{SO}}_4,10{\mathrm{H}}_2\mathrm{O}\text{ . } \end{gathered}$$