Question

If air is 80 percent nitrogen and 20 percent oxygen by volume what is density

Answer 1

1 mol of any gas or mix of gases at STP conditions will have a volume of 22.4 L. Since the problem doesn’t said what are the conditions I will asume that are STP condition and the volume of one mole of the mix will have a volume of 22.4 L.

You may know that density is
D=m/v

In one mole of air I will have 80% of Nitrogen (N2) and 20% oxygen (O2).

So the mass of one mole of air will be

14 x2x0.80+16x2x0.20 = 22.4 g + 6.4 g = 28.8 g

D= 28.8/22.4 = 1.28 g/L

Of course if the temperature is higher the density will be smaller because the volume of one mole will be bigger and viceversa if the temperature decrease. Also if the pressure is different than one atm the volume of a mol will change.