A $2.00$  litre evacuated container has a mass of $1050.0 \mathrm{g}.$. When the container is filled with an unknown gas at $800 \mathrm{mm} \mathrm{Hg}$ pressure and $25.0° \mathrm{C}$ the mass is $1052.4 \mathrm{g}$. What is the molar mass of the gas (in $\mathrm{g} {\mathrm{mol}}^{-1}$ )?

• Gas Mass = (Mass of Container w/ Gas) - (Mass of Empty Container)
• Gas Mass = 1052.4 g - 1050.0 g = 2.4 g Using the mass and number of moles of the unknown gas, we can finally calculate its molar mass:

Molar Mass equals mass / moles
2.4 g / 0.086 mol = 27.9 g/mol for the molar mass Mass of gas = 1052.4 - 1050 = 2.4 g $PV=\frac{W}{M}RT$ $\Rightarrow M=28$