Number of grams of oxygen in 32.2 g Na₂SO₄.10H₂O is

${\mathrm{Na}}_2{\mathrm{SO}}_4\cdot 10{\mathrm{H}}_2\mathrm{O}=2\times 23+32+4\times 16+10\times 18$

=46 + 32 + 64+ 180 - 322 gm

$322 \mathrm{gm} {\mathrm{Na}}_2{\mathrm{SO}}_4\cdot 10{\mathrm{H}}_2\mathrm{O}\text{ contains }=224 \mathrm{gm}\text{ oxygen }$

$32.2 \mathrm{gm}{ \mathrm{Na}}_2{\mathrm{SO}}_4\cdot 10{\mathrm{H}}_2\mathrm{O}\text{ contains }$

$=\frac{32.2\times 224}{322}=22.4\mathrm{gm}$.