Assuming that air contains 78 percent N2, 21 percent O2 and 1 percent Ar, all by volume. How many molecules of each type of gas are present in 1.0 L of air at STP?

First we calculate the number of moles of air in the given volume at STP. At STP, each mole is 22.4 L.

number of moles = (1.0 L) / (22.4 L/mol)

= 5/112 moles

Then, we multiply the determined number of moles by the Avogadro's number to calculate for the number of molecules.

number of molecules = (5/112 moles) (6.022 x 10^23)

= 2.688 x 10^22

Then, we multiply the number of molecules with the decimal equivalent of the percentages.

N2 = (2.688 x 10^22) (0.78) = 2.1 x 10^22 molecules

O2 = (2.688 x 10^22) (0.21) = 5.65 x 10^21 molecules

Ar = (2.688 x 10^22) (0.01) = 2.688 x 10^20 molecules