Mean free path as a function of density in volume and particle size:

assuming a unit volume for KCl, then the density will be:

rou=V0, where V0 is the volume of the sample.

Number of particles will be:

Np=V0/V, where V=4pi/3*R^3 is the volume of the particles,

then Np=V0/(4pi/3*R^3)

The mean free path will be:

lambda=1/simga/Np, where sigma=pi*R^2 is the crossection of the particle.

Therefore lambda=4R/3/rou

Example:

rou=0.01, R=1e-6 meter

then the mean free path is:

lambda=1.3e-4 meter which is 0.13 mm.