How is sulfurous acid named from the sulfite ion?

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The structure of phosphorous acid, H3PO3. — oxyacid The structure of phosphorous acid, H₃PO₃.

When naming an oxyacid derived from an oxyanion, the nomenclature is determined by the suffix of the anion's name.

For anions that end in -ite, such as the sulfite ion ( $S O_{3}^{2 -}$ ), the suffix is replaced by -ous to name the corresponding acid. Following this rule, the acid $H_{2} S O_{3}$ is named sulfurous acid.1