[elle1019]

Helium-oxygen mixtures are used by divers to avoid the bends and are used in medicine to treat some respiratory ailments. What percent (by moles) of helium is present in a helium-oxygen mixture having a density of 0.538 g/L at 25 degrees C and 721mmHg?
Express your answer numerically as a percentage.

I realize that PV = nRT and that it must be manipulated to find the density of oxygen and helium d= PM /RT
However I'm not sure what to do after. Help would be much appreciated. Thank you in advance

[Borek]

Try to express density of a gas as a function of its composition. Start assuming there is 1 mole of gases together, and it contains x moles of He and (1-x) moles of O₂. What is a mass of the mixture (expressed in terms of x and (1-x)? What is its density (hint: what is volume of 1 mole of gas regardless of its composition)?