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 He(g) is present in a helium-oxygen mixture having a density of 0.538g/L at 25celsius and 721mmHg ?

Answer 1

change that density to stp.

density1=P2T2/(P1*T1 density2)

Then

at stp, using decimalpercents,
densityHe*percentHe+densityO2*percentO2 =denstiygasSTP

Those percents will be by mass. To change to percent by moles, divide each percent by mass by atomic mass of He, and O2, and normalize to 100.

Answer 2

wat do u mean by change density to stp