Q:   Ordinary air at room temperature has a density of about 1 kg
             per cubic meter.  Pretend that is made entirely of oxygen
             molecules, each of which has a typical size of 10<sup>-10</sup> m.

       What is the mean free path of molecules in the air?

             First, we need to find the number density of oxygen molecules.

                          1 kg            1 molecule
                   n  =  -------  *  --------------------
                           m^3       32 x 1.67 x 10^(-27) kg


                      =    1.9 x 10^(25) molecules per m^3


              Now we can compute the mean free path

                                           1
              L  =  ---------------------------------------------------------
                    (1.9 x 10^(25) molecules per m^3) * pi * ( 10^(-10) m)^2

 
                 =   2 x 10^(-6) m   =  2 microns