Ka, Kb and Kw are related in a simple equation: Ka multiplied by Kb equals Kw. This equation can be used to determine any of the variables if the other two variables are known.
Ka, or the acid dissociation constant, is an equilibrium constant for the dissociation of acids. Strong acids completely dissociate in water, while weak acids only partially dissociate. The Ka of an acid shows the strength or weakness of an acid. Strong acids have large Ka values because they completely dissociate in water, and weak acids have small Ka values. Kb, or the base dissociation constant, is the equilibrium expression for bases. In water strong and weak bases both establish an equilibrium value. This value is denoted by the Kb value. Kw is the water dissociation constant and is defined as the dissociation and ionization of water. Kw is always 1.0 x 10^-14 when the water is at 25 degrees Celsius. The relationship between Kw, Kb and Ka allows chemistry students to determine the relative strength of an acid or a base by comparing it to the Kw value. For example, if the Ka value of an acid is known, then the Kb value of the acid’s conjugate base can be found by plugging in the Ka and Kw values into the previously mentioned equation.