The mass per cent of different elements present in sodium sulphate, ${\mathrm{Na}}_2{\mathrm{SO}}_4$ respectively are

$\text{ Mass per cent of an element = }=\frac{\text{ Mass of that element in the compound }}{\text{ Molar mass of the compound }}\times 100$

$\text{ Molar mass of }{\mathrm{Na}}_2{\mathrm{SO}}_4 =(2\times 22.99)+32.06+(4\times 16.00)$

                                      = 142.04 g

$\text{ Mass per cent of sodium }=\frac{45.98\times 100}{142.04}=32.37$

$\text{ Mass per cent of sulphur }=\frac{32.06\times 100}{142.04}=22.57$

$\text{ Mass per cent of oxygen }=\frac{64\times 100}{142.04}=45.06$