Question

A solution contains 45 grams of nitrogen gas and 40 grams of argon. What is the mole fraction of argon in this solution?

Answer 1

 0.38 

You first calculate the total moles by dividing the grams by molecular weight: 
45 g N2 / 28.02 g/mol = 1.6 mol N2 
40 g Ar / 39.95 g/mol = 1.0 mol 

Then you divide the moles of Ar by the total number of moles: 
1.0 / (1.6 + 1.0) = 0.38 mol fraction