Question

A gaseous mixture of $He$ and $O_2$ is found to have a density of $0.518g{L}^{-1}$ at ${25}^{\circ }C$ and $720torr$. What is the percent by mass of helium in this mixture?

Answer 1

First determining the average molecular mass and then number of moles of helium and oxygen.
Moles of helium $=0.665$; Moles of oxygen $=0.335$
Mass per cent of helium $=\frac{4\times 0.665}{4\times 0.664+32\times 0.335}\times 100$
$=19.88$