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Dry air (0% humidity) is approximately 78% N2, 21% O2, and 1% Ar by moles. Calculate the molar mass and density of dry air at STP. Assume ideal behavior and use two significant figures in your answers.

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For the first question, we calculate as follows with the basis of 100 mol dry air:

78 28 + 2132 + 139.9 / 100
 
molar mass of air =28.959=28.96
 
For the second question, we calculate as follows:
 
Vapor density = molecular weight/2=14.48
Relative density= vapor density density of hydrogen(0.089). = 1.29