Covered in Section 14.1.3, Fick's Law is like Fourier's law but applied to mass transfer.
Equation 14.12:
Where is diffusive mass flux of species A, is density, is mass diffusion of species A into B, is the mass of species A.
Equation 14.13:
Where is the diffusive molar flux, is molecular concentration, is the diffusion of species A into B, and is the mole fraction of species A.
See ME3304 S24 Lecture Note (8) Mass Transfer for more information.