Problem: For a weak diprotic acid H2X, what is the relationship between [X2-] and Ka2? Under what conditions does this relationship exist?

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We want to know the relationship between the anion [X2-] and Ka2 that is formed by a diprotic acid.


Diprotic acids, such as sulfuric acid (H2SO4), carbonic acid (H2CO3), hydrogen sulfide (H2S), chromic acid (H2CrO4), and oxalic acid (H2C2O4) have two acidic hydrogen atoms. These two acidic hydrogen atoms dissociate stepwise so there are two Kexpressions.


Consider the ionization of a general diprotic acid H2X.

The general Ka expression for acids is:


Ka=productsreactants


H2X(aq) + H2O(l) → H3O+(aq) + HX-(aq)

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For a weak diprotic acid H2X, what is the relationship between [X2-] and Ka2? Under what conditions does this relationship exist?

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