A 0.642 g sample of an unknown gas was collected over water at 25.0 degrees C and 1.04 atm. The collection cylinder contained 151.3 mL of gas after the sample was released. How do you find the molar mass of the unknown gas?
The molar mass of the gas is 103 g/mol.
We can use the Ideal Gas Law to solve this problem.
#color(blue)(bar(ul(|color(white)(a/a)PV = nRTcolor(white)(a/a)|)))" "#
#PV = (m/M)RT#
And we can solve this equation to get
#M = (mRT)/(PV)#
Since you are collecting the gas over water,
At 25.0 °C, the partial pressure of water is 0.0313 atm
Thus, in your problem,