Answer
Diameter is $3.2*10^{-8}cm$
Work Step by Step
Given, $\lambda= Mean free path= 0.80*10^{-5}cm$ and $N/V=2.7*10^{19}molecules/cm^{3}$
We know, $\lambda=\frac{1}{\sqrt 2*pi*d^{2}*N/V}$,
So, $0.80*10^{-5}=\frac{1}{\sqrt 2*3.14*d^{2}*0.8*2.7^{19}}$
which gives, $d=3.2*10^{-8}cm$
