acid dissociation constant

acid dissociation constant or acidity constant symbol: K_a; the thermodynamic equilibrium constant for the dissociation of an acid. For a dilute solution of a weak acid, HA, dissociating in water according to the equilibrium:

HA + H₂O  ↔ H₃O⁺ + A^–,
K_a = (a_H3O+ × a_A–)/a_HA

where a is the activity of the species designated by the subscripts. The activity of the water has been omitted from the equation since it may be taken as unity for a dilute aqueous solution. K_a is a measure of the strength of the acid, i.e. of its ability to donate hydrons to water.