How to calculate molecular weight of air

Well, all you can do is approximate it.

What is the composition of air?
Nitrogen gas 78%
Oxygen 21%

An approximate molecular weight would be the average of those two gases which are the most abundant.

78% is of mass 28 g
21% is of mass 32 g.

100% would have approximately \frac{(78% \times 28) + (21% \times 32)}{78% + 21%} \times 100% \approx 28.8