How does size of molecule affect diffusion?

Hint: To solve this we must know the Graham’s law of diffusion. Graham’s law of diffusion states that at constant temperature and pressure, the atoms with high molecular mass effuse or diffuse slower than the atoms with low molecular mass. Graham’s law thus states that the square root of the molecular mass is inversely proportional to the rate of diffusion.

Complete answer:
We know that the Graham’s law of diffusion states that at constant temperature and pressure, the atoms with high molecular mass effuse or diffuse slower than the atoms with low molecular mass. Graham’s law thus states that the square root of the molecular mass is inversely proportional to the rate of diffusion.
The expression for the Graham’s law of diffusion is as follows:
$\dfrac{{{r_1}}}{{{r_2}}} = \dfrac{{\sqrt {{M_2}} }}{{\sqrt {{M_1}} }}$
Where, ${r_1}$ and ${r_2}$ are the rates of diffusion,
${M_1}$ and ${M_2}$ are the molecular masses.
We know that higher the molecular mass bigger is the size of the molecule.
The expression for the Graham’s law of diffusion shows that the rate of diffusion is inversely proportional to the molecular mass i.e. the rate of diffusion is inversely proportional to the size of the molecule.
Thus, we can say that as the size of the molecule increases the rate of diffusion of the molecule decreases and vice versa.

Note: Graham’s law is more accurate for the molecular effusion that involves the movement of one gas at a time. For diffusion, Graham’s law is approximate. This is because the process of diffusion involves movement of more than one gas at a time.