A pre-weighed vessel was filled with oxygen at NTP and weighed. It was then evacuated, filled with ${\mathrm{SO}}_2$ at the same temperature and pressure and again weighed. The weight of oxygen will be

An equal volume of gas contains an equal number of moles at constant temperature and pressure.

So, Moles of ${\mathrm{O}}_2=$ Moles of ${\mathrm{SO}}_2$

$1 \mathrm{mole} {\mathrm{O}}_2=32 \mathrm{g}$ and $1 \mathrm{mole} {\mathrm{SO}}_2=64 \mathrm{g}$

Hence, weight of ${\mathrm{O}}_2=\frac{1}{2}$ that of ${\mathrm{SO}}_2$.