How can the number of oxygen atoms help predict the strength of an oxyacid?

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

For a given central nonmetal atom, the strength of an oxyacid increases as the number of oxygen atoms increases. This is because adding more oxygen atoms raises the oxidation number of the central atom.1

As the oxidation number of the central atom increases, the acid becomes stronger. For example, sulfuric acid ( $H_{2} S O_{4}$ ), where sulfur has an oxidation state of +6, is a stronger acid than sulfurous acid ( $H_{2} S O_{3}$ ), where sulfur has an oxidation state of +4. Similarly, nitric acid ( $H N O_{3}$ ) is stronger than nitrous acid ( $H N O_{2}$ ) because the nitrogen atom in nitric acid has a higher oxidation number.1