# Plate Theory and Extensions - The Plate Theory > Page 5

Bearing in mind, the change in mass will be the product of concentration and change in volume; thus, the change of mass of solute (dm) in plate (p) will be

dm = (X_{m(p-1)} - X_{m(p)})dV
(3)

Now, as
equilibrium is maintained in the plate (p) by definition, the mass (dm) will be
distributed between the two phases, resulting in a solute concentration change
of dX_{m(p)} in the mobile phase and dX_{s(p)} in the stationary
phase_{. Then,}

_{ }

_{
} d_{m} = v_{s}dX_{s(p) }+ v_{m}dX_{m(p) }_{ (4)}

_{ }

Substituting
for dX_{s(p)} from equation (2),

d_{m} = (v_{m} + Kv_{s})dX_{m(p) }_{ (5)}