Final answer:
Phosphorous acid (H₃PO₃) is a diprotic acid as it can only donate two protons upon ionization, in contrast to phosphoric acid (H₃PO₄) which is triprotic.
Step-by-step explanation:
The question asks if phosphorous acid (H₃PO₃) is a diprotic oxyacid. In the structure of phosphorous acid, only the two hydrogen atoms that are bonded to an oxygen atom are acidic. According to the information provided, phosphorous acid is indeed considered a diprotic acid because it can donate two protons (H+ ions) in aqueous solution. Although the formula may suggest that it is triprotic, in reality, only two of the hydrogens are in a position to be lost as protons.
In contrast, phosphoric acid (H₃PO₄), which is sometimes confused with phosphorous acid, is a triprotic oxyacid and can donate three protons. The ionization of phosphoric acid is a stepwise process where each dissociation step involves the release of one H+ ion, and the constant differences of ionization indicate that each step becomes progressively weaker.
In summary, phosphorous acid (H₃PO₃) is a diprotic acid, not triprotic, which means that it can only undergo two ionizations to donate two protons.