Strength of Acids and Bases

The strength of an acid or base is set by how much $H_3O^+$ or $OH^-$ it produces per mole dissolved in water: a strong acid/base is almost completely dissociated, while a weak acid/base is only partially dissociated, leaving an equilibrium between the unionised molecule and its ions. For the general ionisation $HA + H_2O \rightleftharpoons H_3O^+ + A^-$, the equilibrium constant is $K = \dfrac{[H_3O^+][A^-]}{[HA][H_2O]}$; since water is present in huge excess and its concentration barely changes, it is absorbed into a new constant $K_a = \dfrac{[H_3O^+][A^-]}{[HA]}$, the ionisation (dissociation) constant of the acid, which directly measures acid strength. Strong acids such as $HCl$ and $HNO_3$ are almost completely ionised and so have a high $K_a$ (e.g. $K_a$ for $HCl$ at $25^\circ C$ is about $2\times10^6$); weak acids such as formic acid ($K_a=1.8\times10^{-4}$) and acetic acid ($K_a=1.8\times10^{-5}$) are only partially ionised. As a rule of thumb, acids with $K_a$ greater than ten are considered strong and those with $K_a$ less than one are considered weak. Because a strong acid's dissociation lies almost completely to the right, its conjugate base has only a negligible tendency to re-accept a proton -- so the conjugate base of a strong acid is a weak base, and vice versa.

Relative strength of conjugate acid-base pairs. A reference table ranking common Bronsted acids from strongest to weakest alongside their conjugate bases (which therefore rank weakest to strongest as bases), built from the unit's ordering $HClO_4 > HCl > H_2SO_4 > HNO_3 > H_3O^+ > HNO_2 > HF > CH_3COOH > NH_4^+ > H_2O$.