1) Find the mean free path of a molecule (= average distance between collisions) of a gas $A$ with $[A]\frac{mol}{m^3}$ and diameter of the molecules $d$ at temperature $T$ and pressure $P$.
If $z$ is the number of collisions per second, then the average time between collisions is $\frac{1}{z}$
If $\lt v \gt$ is the average speed, then the average distance between collisions is $\lt v \gt \frac{1}{z} $
with (see → here)
$z$=
$\frac{P}{k\cdot T}\pi d^2 \sqrt2 \sqrt{\frac{8RT}{\pi\cdot M}}=$
and
$\lt v \gt =$
$\sqrt{\frac{8RT}{\pi\cdot M}}$
and so:
Mean free path=
$\frac{kT}{\sqrt2\cdot P\pi d^2}$
2) Use the formula found for calculating the mean free path of a nitrogen molecule at $20^oC$ and $1 \;bar$ (diameter of a molecule = $3,64\cdot 10^{-10}m$ )
