[Shing]

A sample of air is made up of nitrogen, oxygen and argon in mole fractions 78% N₂, 21% O₂, 1.0% Ar, what is the density of air at STP?

What I did..
D = PM/RT where M is molar mass.

denisty of N2 using the above formula was: 1.25 g/l
denisty of O2 using the above formula was: 1.42 g/l
denisty of Ar using the above formula was: 1.74 g/l

so with mole fractions: (78 * 1.25) + (21 *1.42) + (1 * 1.74) = 129.01 g/l ( denisty of air)

is the process right? (ignore any incorrect rounding)

[Yggdrasil]

78% = 0.78, not 78.

[Shing]

78% = 0.78, not 78.

so the process is right? except the 0.7...

[Yggdrasil]

Yes, your process is correct.  Your answer is just off by a factor of 100.

[Shing]

Yes, your process is correct.  Your answer is just off by a factor of 100.

Thanks a million