The density of air is 1.28g/liter (given)
now 1 liter =1000cm^3
therefore density of air=1.28/1000 g/cm^3
that is = 0.0128g/cm^3
we know 1kg = 1000g
therefore 1g=0.001kg
1 m^3 = 1000liters
therefore 1liter =0.001m^3
therefore the density of air=1.28 *.001kg/ .001m^3
the density of air = 1.28kg /m^3