A 0.642 g sample of an unknown gas was collected over water at 25 C and 1.04 atm. The collection cylinder contained 151.3 mL of gas after the sample was released. Find the molar mass of the unknown gas?

Answer 1

Convert the volume to V at 1 atm.
At 25°C, the molar volume is 24.465 L.

Use proportion to find the molar mass, M
0.642/V = M/24.465

Answer 2

Since this gas was collected over H2O, I think we should correct for the vapor pressure of H2O at 25C which is 23.8 mm Hg. That makes the pressure of the gas 1.0086 atm which is too many s.f. but I just leave that in my calculator. That makes a difference of 3 or 4 in the molar mass of the gas vs the problem as solved above.
If you want to work the problem with chemistry, use PV = nRT and solve for n.
1.0086 x 0.1513 = n*0.08206*298
Then molar mass = 0.642/n