Question

how many molecules of O2 are present in 1l of air containing 80% volume of O2 at STP?

Answer 1

Assuming Air and Oxygen to be an Ideal gas at STP. 

In 1 liter of air, 0.80 liter of Oxygen.

1 Mole of O2 occupies 22.4 liters at STP.
So   in 0.80 liter of Oxygen, we have   0.8 * 1 /22.4  moles = 1/28  of a mole

In one mole there are 6.023 * 10²³  molecules of Oxygen .  (Avogadro number )

So number of molecules :  6.023 * 10²³ / 28 
        ≈ 2.15 * 10²²  

Answer 2

22.4l of air at STP contains 6.023*10^23 molecules

80% of 1l = 80/100*1 = 0.8l

0.8l of air will contain = 6.023*10^23 * 0.8/22.4 
                                  = 21.5*10^21 molecules