JEE Main · 2016 · 03 Apr 2016 · Offline · Redox Reactions and Electrochemistry

Oxidation state of phosphorus is fixed by its bonds. In orthophosphorous acid $\mathrm{H}{}_3\mathrm{PO}{}_3$ the phosphorus has one P--H bond, one P=O and two P--OH bonds, which works out to $+3$. In pyrophosphorous acid $\mathrm{H}{}_4\mathrm{P}{}_2\mathrm{O}{}_5$ each phosphorus is likewise $+3$. Hypophosphoric acid has phosphorus at $+4$ and pyrophosphoric acid at $+5$, so the pair with both atoms at $+3$ is orthophosphorous and pyrophosphorous acids.