Concept preview

What you will be able to do

1. 01Write Ka and Kb expressions
2. 02Calculate [H⁺] and pH of a weak acid, checking the approximation
3. 03Relate Ka and Kb for a conjugate pair and use pKa

Because a weak acid exists at equilibrium, $\mathrm{HA \rightleftharpoons H^+ + A^-}$, it has its own equilibrium constant called the acid ionization constant, Ka. Written in the usual way — products over reactant, the pure liquid water omitted — it is $K_a = \dfrac{[\mathrm{H^+}][\mathrm{A^-}]}{[\mathrm{HA}]}$. The size of Ka measures how far the ionization proceeds: a larger Ka means more ionization and hence a stronger (though still weak) acid, while a very small Ka means the acid barely ionizes.