An evacuated glass vessel weights 40.0 g when empty, 135.0 g when filled with a liquid of density $0.95 \mathrm{~g} \mathrm{~mL}^{-1}$ and 40.5 g when filled with an ideal gas at 0.82 atm at 250 K . The molar mass of the gas in $\mathrm{g} \mathrm{mol}^{-1}$ is: (Given : $\mathrm{R}=0.082 \mathrm{~L} \mathrm{~atm} \mathrm{~K}^{-1} \mathrm{~mol}^{-1}$ )