Question

A 3g sample of ${\mathrm{Na}}_2{\mathrm{CO}}_3.10{\mathrm{H}}_2\mathrm{O}$ is heated to a constant mass. How much anhydrous salt remains?

Answer 1

Step-1 - Calculate the molar mass of anhydrous salt

Molecular mass of $N{a}_{2}C{O}_3=2\times 23+12+3\times 16=106g$

Molecular mass of $H_{2}O$$=\left(1\times 2+16\right)=18\mathrm{g}$

Step-2 Calculate the mass fraction

Mass fraction of $H_{2}O$ in $N{a}_{2}C{O}_3$= $\frac{18}{106}=0.17$

$or0.83N{a}_{2}C{O}_{3}bymass$

As given, the sample mass of $N{a}_{2}C{O}_3$$=3g$

After removing water, the mass of Anhydrous salt $=3\times 0.83=2.5g$

Hence, 2.5g of anhydrous salt remains.