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?

Options

A. 19.95%
B. 39.90%
C. 9.95%
D. 30%

Answer

A

Explanation

We know that, `Pm = dRT`
`(720)/(760) xx m = 0.518 xx 0.0821 xx 298`
`m = 13.37 g//mol`
Let mole fraction of He in mixture be `alpha`.
`:. X_(He) = alpha, x_(O_(2)) = (1 - alpha)`
Average `mw = alpha xx M_(He) + (1 - alpha) M_(O_(2))`
`13.37 = alpha xx 4 + (1 - alpha) 32`
`alpha = 0.666`
% by mass of He`= 0.666 xx (4)/(0.666 xx + 0.334 xx 32) xx 100`
`= 19.95%`