# Why is acetic acid a weak acid?

It ionizes to a small extent in water to form a low concentration of ${H}^{+}$ ions (or ${H}_{3} {O}^{+}$) in solution.
In the reversible chemical equation, $C {H}_{3} C O O H \to {H}_{a q}^{+} + C {H}_{3} C O {O}_{a q}^{-}$,
the equilibrium lies far to the left and hence the ${K}_{a}$ value is very small.