$\frac{P_{1}}{\rho_{1} T_{1}}=\frac{P_{2}}{\rho_{2} T_{2}}$

Given that Density at NTP is $\rho_{2}=1.2 \; g / L$

$T _{2}=20^{\circ} C =293.15 K$ and $P _{2}=76 \; cm$ of $Hg$

We have $T _{1}=21^{\circ} C =294.15 \; K$ and $P _{1}=71.8 \; cm$ of $Hg$

Thus $\rho_{1}=\rho_{2} \times \frac{ T _{2}}{ T _{1}} \times \frac{ P _{1}}{ P _{2}}=1.2 \times \frac{293.15}{294.15} \times \frac{71.8}{76}=1.13 \; g / L$

Thus mass of the gas $=\rho_{2} \times V_{2}=1.13 \; g / L \times 1 L =1.13 \; g$