Question

Stoichiometry and POAC

R.C MUKHERJEE

Q 14 Anhydrous Sodium sulphate can absorb H2O vapour and be converted to d decahydrate . By how many grams would the mass of a 1g sample of d thoroughly dried Na2SO4 increase if exposed to sufficient H2O vapour to be converted to decahydrate.?

Answer 1

Hydration of sodium sulphate can be represented by the following equation :

$$\begin{gathered} {\mathrm{Na}}_2{\mathrm{SO}}_4+10{\mathrm{H}}_2\mathrm{O}\to {\mathrm{Na}}_2{\mathrm{SO}}_4.10{\mathrm{H}}_2\mathrm{O} \\ \mathrm{Molar}\mathrm{mass}\mathrm{of}{\mathrm{Na}}_2{\mathrm{SO}}_4=23\times 2+32+16\times 4=142\mathrm{g}/\mathrm{mol} \\ \mathrm{Molar}\mathrm{mass}\mathrm{of}{\mathrm{Na}}_2{\mathrm{SO}}_4.10{\mathrm{H}}_2\mathrm{O}=23\times 2+32+16\times 4+10(2\times 1+16)=322\mathrm{g}/\mathrm{mol} \\ \mathrm{Thus}\mathrm{when}142\mathrm{g}{\mathrm{Na}}_2{\mathrm{SO}}_4\mathrm{absorbs}\mathrm{water}322\mathrm{g}\mathrm{of}\mathrm{hydrated}\mathrm{product}\mathrm{is}\mathrm{obtained}. \\ \mathrm{Therefore}1\mathrm{g}{\mathrm{Na}}_2{\mathrm{SO}}_4\mathrm{will}\mathrm{absorb}\mathrm{water}\mathrm{to}\mathrm{give}=\frac{322}{142}\mathrm{g}\mathrm{product} \\ =2.27\mathrm{g}\mathrm{product} \\ \mathrm{Increase}\mathrm{in}\mathrm{weight}\mathrm{of}{\mathrm{Na}}_2{\mathrm{SO}}_4\mathrm{upon}\mathrm{hydration}=2.27\mathrm{g}-1\mathrm{g} \\ =1.27\mathrm{g} \end{gathered}$$
Thus there will be an increase of 1.27 g in the weight of anhydrous sodium sulphate when it absorbs water to form sodium sulphate decahydrate.