In the usual notation, the mean free path of a gas molecule,
\(\lambda\) = \(\frac{1}{\sqrt2\pi d^2(N/V)}\)
Density, ρ, of the gas = \(\frac{mass}{volume}\)
= \(\frac{mN}{V}\), where m is the molecular mass.
∴ \(\frac{N}{V}\) = \(\frac{ρ}{m}\)
∴ λ = \(\frac{m}{\sqrt2\pi d^2ρ}\)