Question

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?

Answer 1

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