How would you find the molecular weight of an unknown gas?
I would determine the mass of a fixed volume of the gas at a known temperature and pressure and then use the Ideal Gas Law to calculate the molar mass.
The mass of an evacuated 255 mL flask is 143.187 g. The mass of the flask filled with an unknown gas at 25.0 °C and 265 Torr is 143.289 g.
The Ideal Gas Law is
#color(blue)(|bar(ul(color(white)(a/a)PV = nRT color(white)(a/a)|)))" "#
#color(blue)(|bar(ul(color(white)(a/a)PV = m/MRTcolor(white)(a/a)|)))" "#
We can rearrange this to get
#M = (mRT)/(PV)#
The molar mass is 28.1 g/mol.
∴ The molecular mass is 28.1 u.