What is the mole fraction of oxygen in a gaseous mixture containing 25 grams of oxygen, 15 grams of nitrogen, and 10 grams of helium?

Convert all the gases from grams to moles using their molar masses first. Remember that nitrogen and oxygen exist as DIATOMIC gases.

O2 - 25 g / 32 g/mol = 0.78 mol O2
N2 - 15 g / 28.02 g/mol = 0.54 mol N2
He - 10 g / 4.0 g/mol = 2.5 mol He

Add all the moles of gases.

0.78 + 0.54 + 2.5 = 3.82 moles

Divide O2 moles by total moles.

0.78/3.82 = 0.20

Ok where is the image ?