Question

What is the density of atmosphere, assuming N2 : 79% and O2 : 21% ?
My chemistry instructor gave us this question to answer, and I feel as if its a trick question.
Might it have something to do with the ideal gas law?

Answer 1

In 100g of air:

$n(O2)=\dfrac{21g}{32g/mol}=0.66 moles$

$n(N2)=\dfrac{79g}{28g/mol}=2.82moles$

$0.66moles+2.82moles=3.48 moles$

$V=\dfrac{nRT}{P}=\dfrac{3.48*0.0821*273K}{1atm}=78L$

$density=\dfrac{m}{V}=\dfrac{100g}{78L}=1.282g/L=1.282kg/m^{3}$