A gasesous mixture of helium and oxygen is found to have a density of `0.518 g dm^(-3)` at `25 ^(@)C` and `720` torr. What is the precent by mass of helium in this mixture?

Answer

`19.88`

Explanation

First determine the average molecular mass and then number of moles of helim = 0.665, Moles of oxygen = 0.335
Mass per cent of helium `= (4 xx 0.665)/(4 xx 0.665 + 32 xx 0.335) xx 100`
`= 19.88`