I am trying to figure out what percent (by moles) of He is present in a helium-oxygen mixture having a density of 0.528g/L at 25C and 721mmHg (torr).
You have found the molar mass of the mixture as 13.6 g/mol.
The molar mass of He is 4 g/mol.
The molar mass of O2 is 2 * 16 = 32 g/mol
Now assume there ...
It finds the mole percentage of the helium-oxygen mixture given its density. The solution is detailed and has a '5/5' rating.