- Molecular diffusion of A in the direction of decreasing concentration (i.e. from left to right)
- Net diffusion of B in the opposite direction
- Diffusion continues until the concentration is uniform throughout

**// Fick’s Law**

The molecular diffusion flux of component A in the z-direction in a mixture of A and B, is given by Fick’s law:

Where,

J_{A} = rate of molecule diffusion, kmol/(m^{2}.s)

D_{AB} = diffusion coefficient (or diffusivity) of A in a mixture of A and B, m^{2}/s

dc_{A}/dz = concentration gradient of A, kmol.m^{-3}/m

The concentration gradient is the driving force, as temperature gradient in Fourier’s law for heat flux by conduction

For gases , Fick‟s law is expressed in terms of partial pressure of component A , pA .

The ideal gas law gives:

Corresponding to a concentration of A,

With the derivative:

**// Diffusion with bulk of mass in motion**

- With uniform concentration of A, the molar flux of A past a stationary observer is
**c**._{A}V

•A concentration gradient dc_{A}/dz gives an additional flux due to molecular diffusion, J_{A}, or total molar flux of A.

// **Diffusion of A through a stagnant B**

Also known as Stefan diffusion

At point 1, p_{A1}is the vapour pressure of A at the temperature of the bath, and at point 2, p_{A2} may be close to zero.