# 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?

##### 1 Answer

The molar mass of the gas is 103 g/mol.

#### Explanation:

We can use the Ideal Gas Law to solve this problem.

#color(blue)(bar(ul(|color(white)(a/a)PV = nRTcolor(white)(a/a)|)))" "#

Since

#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,

∴