# Plate Theory and Extensions - The Plate Theory > Page 4

Differentiating equation (1),

dX_{s }= KdX_{m} (2)

Consider three consecutive plates in a column, the (p-1), the (p) and the (p+1) plates and let there be a total of (n) plates in the column. The three plates are depicted in Figure 2.

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**Figure 2. Three Consecutive
Theoretical Plates in a Column**

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Let the
volumes of mobile phase and stationary phase in each plate be (v_{m})
and (v_{s}) respectively, and the concentrations of solute in the
mobile and stationary phase in each plate be X_{m(p-1), }X_{s(p-1)},
X_{m(p)}, X_{s(p)}, X_{m(p+1)}, and X_{s(p+1), }respectively.
Let a volume of mobile phase, dV, pass from plate (p-1) into plate (p) at the
same time displacing the same volume of mobile phase from plate (p) to plate
(p+1). As a result, there will be a change of mass (dm) of solute in plate (p)
which will equal the difference in the mass entering plate (p) from plate (p-1)
and the mass of solute leaving plate (p) and entering plate (p+1). It is now
possible to apply a simple mass balance procedure to the plate (p).