If we consider the motion of all the molecules, the expression of mean free path is (n is the no. of molecules per unit volume and d is the diameter of each molecule)

The mean free path (λ) of gas molecules is the average distance a molecule travels between collisions. In a simplified model, considering ideal gas behavior and neglecting molecular size, the expression for mean free path is:

$\frac{\mathrm{kT}}{\sqrt{2}{\mathrm{\pi d}}^{2}P}=\frac{1}{\sqrt{2}{\mathrm{n\pi d}}^2}$

Where:

• k is the Boltzmann constant,
• T is the temperature in Kelvin,
• d is the diameter of a gas molecule,
• P is the pressure.
• n is the no of molecules per unit volume

It's important to note that this expression assumes a simple model of gas molecules as point particles and neglects molecular size, intermolecular forces, and other complexities. In reality, molecular size and interactions play a significant role in determining the mean free path.