At 300 K and 760 torr pressure, the density of a mixture of He and $\mathrm{O}_2$ gases is $0.543 \mathrm{gL}^{-1}$. The mass percent of oxygen approximately is $$ \left(R=0.0821 \mathrm{~L} \mathrm{~atm} \mathrm{~K}^{-1} \mathrm{~mol}^{-1}\right) $$