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