Gas Mixtures and Dalton's Law

[runnerchck917]

A 2.00 L flask at 315K contains a mixture of He and oxygen gas with a pressure of 1atm. If the mole fraction of oxygen gas is .60, what is the mass fraction of oxygen gas?

[kingchemist]

Not sure if you need the volume or pressure. If mole fraction O2 = 0.6, then mole fraction He = 0.4
If O=16 and He=4, then O2 = 32.
0.6 moles of O2 at 32g = 19.2g
0.4 moles He at 4g = 1.6g

Total mass = 20.8g
Mass fraction O2 = 19.2/20.8

Otherwise you could find the total moles in the flask from PV=nRT. Then use mole fractions to find partial pressures of each gas. Then find moles of each gas present, then mass and then mass fraction. Think you might get the same answer - perhaps worth checking