The governing equations for diffusion mass transfer are [34]
| (277) | 
and
|  | (278) | 
where
| (279) | 
and
| (280) | 
In these equations 
![]() is the mass flux of species A,
 is the mass flux of species A, 
![]() is the mass diffusivity,
is the mass diffusivity, ![]() is the mass fraction of species A and
 is the mass fraction of species A and ![]() is the density of species A. Furthermore,
is the density of species A. Furthermore, ![]() is the rate of increase
of the mass of species A per unit volume of the mixture. Another way of
formulating this is:
 is the rate of increase
of the mass of species A per unit volume of the mixture. Another way of
formulating this is:
| (281) | 
and
|  | (282) | 
where
|  | (283) | 
and
| (284) | 
Here, 
![]() is the molar flux of species A,
 is the molar flux of species A, 
![]() is the mass diffusivity,
is the mass diffusivity, ![]() is the mole fraction of species A and
 is the mole fraction of species A and ![]() is the molar concentration of species A. Furthermore,
is the molar concentration of species A. Furthermore, ![]() is the rate of increase
of the molar concentration of species A.
 is the rate of increase
of the molar concentration of species A.
The resulting equation now reads
|  | (285) | 
or
|  | (286) | 
If ![]() and
 and ![]() are constant, these equations reduce to:
 are constant, these equations reduce to:
|  | (287) | 
or
|  | (288) | 
Accordingly, by comparison with the heat equation, the correspondence in Table (16) arises.