Equilibrium acid–base reaction:

pK_a:

Acids can be explained as being weak or strong and the pK_a is a measure of this. Dissolving acetic acid in water causes in equilibrium among the carboxylic acid and the carboxylate ion shown in the below diagram.

Figure:   Equilibrium acid-base reaction of ethanoic acid with water.

Ethanoic acid on the left side of the equation is known as the free acid, whereas the carboxylate ion created on the right side is known as its conjugate base. The extent of ionization or dissociation is described by the equilibrium constant (K_eq);

K_eq = [PRODUCTS] / [REACTANTS]

       = [CH₃CO₂^-] [H₃O⁺]/[CH₃CO₂H][H₂O]

K_eq is generally measured in a dilute aqueous solution of the acid and thus the concentration of water is high and assumed to be constant. Hence, we can rewrite the equilibrium equation in a simpler type in which K_a is the acidity constant and involves the concentration of pure water (55.5 M).

K_a = K_eq [H₂O]

      = [CH₃CO₂^-] [H₃O⁺]/[CH₃CO₂H]