Ionisation of Weak Acids

Weak acids are only partially dissociated in water, leaving an equilibrium between the undissociated acid and its ions. For the ionisation of a weak monobasic acid $HA + H_2O \rightleftharpoons H_3O^+ + A^-$, the equilibrium constant is $K_C=\dfrac{[H_3O^+][A^-]}{[HA][H_2O]}$. Since water is present in large excess in a dilute solution, its concentration can be folded into a constant $K$, giving $K_C \cdot K = \dfrac{[H^+][A^-]}{[HA]}$ (writing $H_3O^+$ simply as the hydrated $H^+$ for convenience); the product of the two constants defines a new constant $K_a=\dfrac{[H^+][A^-]}{[HA]}$, the dissociation constant of the acid, which -- like all equilibrium constants -- …