Question

What is the average molecular mass in dry air (78% N₂ and 22%O₂​​​​​​)

Answer 1

Dry air is a mixture of gases where the molecular weight can be calculated by adding the weight of each component.As shown below:
Wt of each component is calculated by multiplying the % wt. of each component with moleculat wt.
(0.78) x (28 g N2/mol) + (0.21) x (32 g O2/mol)) + (0.009) x (40 g Ar/mol) + (0.001) x (44 g CO2 /mol)

= 21.84 + 6.72 + 0.36 +0.044 = 28.964 ​g/mol

molecular weight /mass of air =28.964 ​g/mol​