The “Ka” stands for the acid dissociation constant. It is the equilibrium constant of the dissociation reaction of an acid. Ka is commonly expressed in the units of mol/L.
There are tables of the acid dissociation constant. In the case of an aqueous solution, the general form of equilibrium reaction is:
HA + H2O ⇆ A- + H3O+
Here HA is an acid that dissociates in the conjugate base of A–
A hydrogen ion combines water to form the hydronium ion H3O+
When the concentrations of HA, H3O+, and A– no longer changed over time, then the reaction is said to be at equilibrium, and the dissociation constant for it may be calculated as:
Ka = [A- ] [H3O+ ] / [HA] [ H2O]
You must be wondering why we have used the brackets here. The square brackets indicate the concentration.
Unless an acid is tremendously concentrated, the equation is simplified by holding the water concentration as constant. Let us see how we can represent it.
HA ⇆ A- + H+
Ka = [A- ] [H+ ] / [HA]
The acid dissociation constant is also referred to as acid ionization constant or acidity constant.