Since air is a mixture, it does not have a "molar mass." However, for calculation purposes, it is possible to speak of its "effective molar mass." (An effective molar mass is a weighted average of the molar masses of a mixture's components.) If air at STP has a density of 1.285 g/L, its effective molar mass is g/mol.
A) 34.4
B) 30.0
C) 26.9
D) 28.8
E) 31.4

Question

Quizplus · Accepted Answer

The density of air at STP is 1.285 g/L. Using the ideal gas law, we can calculate the number of moles of air in one liter at STP:
PV = nRT
(1 atm) (1 L) = n (0.0821 L atm/mol K) (273 K)
n = 0.0448 mol
The mass of air in one liter is therefore:
mass = n x Molar Mass
1.285 g = 0.0448 mol x Molar Mass
Molar Mass = 28.8 g/mol
Therefore, the effective molar mass of air at STP is 28.8 g/mol. The answer is D.

Since Air Is a Mixture, It Does Not Have a "Molar