A careful study of such solutions has shown that there is a definite state of equilibrium between the constituents of the solution; that is, there is a definite relation between the undissociated acetic acid and its ions, which is characteristic for the prevailing conditions.  It is not, however, assumed that this is a condition of static equilibrium, but rather that there is continual dissociation and association, as represented by the opposing reactions, the apparent condition of rest resulting from the fact that the amount of change in one direction during a given time is exactly equal to that in the opposite direction.  A quantitative study of the amount of undissociated acid, and of H^{+} ions and C_{2}H_{3}O_{2}^{-} ions actually to be found in a large number of solutions of acetic acid of varying dilution (assuming them to be in a condition of equilibrium at a common temperature), has shown that there is always a definite relation between these three quantities which may be expressed thus: 

(!Conc’n H^{+} x Conc’n C_{2}H_{3}O_{2}^{-})/Conc’n HC_{2}H_{3}O_{2} = Constant!.

In other words, there is always a definite and constant ratio between the product of the concentrations of the ions and the concentration of the undissociated acid when conditions of equilibrium prevail.

It has been found, further, that a similar statement may be made regarding all reversible reactions, which may be expressed in general terms thus:  The rate of chemical change is proportional to the product of the concentrations of the substances taking part in the reaction; or, if conditions of equilibrium are considered in which, as stated, the rate of change in opposite directions is assumed to be equal, then the product of the concentrations of the substances entering into the reaction stands in a constant ratio to the product of the concentrations of the resulting substances, as given in the expression above for the solutions of acetic acid.  This principle is called the !Law of Mass Action!.

It should be borne in mind that the expression above for acetic acid applies to a wide range of dilutions, provided the temperature remains constant.  If the temperature changes the value of the constant changes somewhat, but is again uniform for different dilutions at that temperature.  The following data are given for temperatures of about 18 deg.C.[1]

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===================
|          |                  |                  |
Molal     | fraction | molal CONCENTRA- | molal CONCENTRA- | value of
concentration | ionized  | TION of H^{+} and| TION of UNDIS-   | constant
constant    |          | acetate^{-} ions | SOCIATED acid    |
______________|__________|__________________|_______________

___|__________
|          |                  |                  |
1.0      |   .004   |       .004       |     .996         | .0000161