m1 = 0.002;   
m2 = 0.028;
nAv = 6.022*10^23;
mu =  (m1/
            nAv)*(m2/nAv)/((m1/nAv) + (m2/nAv))

kB = 1.38065*10^-23;
temp = 628;
meanSpeed = (8*kB*temp/Pi/mu)^0.5

d12 = 
      0.5 (2.827*10^-10 + 
            4.163*10^-10)
sigma = 0.5 (0.27*10^-18 + 0.64*10^-18)

k01 = 
      nAv*d12^2*(8*Pi*kB*temp/mu)^0.5*1000
k02 = nAv*sigma*meanSpeed*1000

So, the fraction of effective collisions is here, as in Exercise 2, of the order of 1.24*(10^6)/7*(10^11)= (10^(-6)).