For a weak diprotic acid H_{2}X, what is the relationship between [X^{2-}] and K_{a2}?
Under what conditions does this relationship exist?

We want to know the relationship between the anion [X^{2-}] and K_{a}_{2} that is formed by a diprotic acid.

**Diprotic acids**, such as sulfuric acid (H_{2}SO_{4}), carbonic acid (H_{2}CO_{3}), hydrogen sulfide (H_{2}S), chromic acid (H_{2}CrO_{4}), and oxalic acid (H_{2}C_{2}O_{4}) have two acidic hydrogen atoms. These two acidic hydrogen atoms dissociate stepwise so there are two K_{a }expressions.

Consider the ionization of a general diprotic acid H_{2}X.

The general ** K_{a} expression** for acids is:

$\overline{){{\mathbf{K}}}_{{\mathbf{a}}}{\mathbf{=}}\frac{\mathbf{products}}{\mathbf{reactants}}}$

H_{2}X_{(aq)} + H_{2}O_{(l)} → H_{3}O^{+}_{(aq)} + HX^{-}_{(aq)}

Diprotic Acid