A gas mixture composed of helium and argon has a density of 0.846g/L at a 734mmHg and 298 K.

What is the composition of the mixture by volume?

1 answer

The general gas law is PV = nRT. n = grams/molar mass which makes this come out to be PV = gRT/M and this can be rearranged to P = gRT/MV where M is molar mass or P*M = gRT. Substitute and solve for M = molar mass. I get about 21 but you need a more accurate number than that estimate.
The let X = fraction He atomic mass 4.003
and 1-X = fraction Ar = atomic mass 39.95
X(4.003) + (1-X)*39.95 = approx 21
Solve for X and 1-X to fractions and multiply by 100 to convert to %.