(!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°C.[1]
========================================================================== | | | | 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 | | | | 0.1 | .013 | .0013 | .0987 | .0000171 | | | | 0.01 | .0407 | .000407 | .009593 | .0000172 | | | | ===========================================================================
[Footnote 1: Alexander Smith, !General Inorganic Chemistry!, p. 579.]
The molal concentrations given in the table refer to fractions of a gram-molecule per liter of the undissociated acid, and to fractions of the corresponding quantities of H^{ +} and C_{ 2}H_{ 3}O_{ 2}^{ -} ions per liter which would result from the complete dissociation of a gram-molecule of acetic acid. The values calculated for the constant are subject to some variation on account of experimental errors in determining the percentage ionized in each case, but the approximate agreement between the values found for molal and centimolal (one hundredfold dilution) is significant.