The equivalent weight of H₃PO₂, when it disproportionates into PH₃ and H₃PO₃ is

$$\begin{gathered} 3{\mathrm{H}}_3\stackrel{+1}{\mathrm{P}}{\mathrm{O}}_2\to \stackrel{-3}{\mathrm{P}}{\mathrm{H}}_3+2{\mathrm{H}}_3\stackrel{+3}{\mathrm{P}}{\mathrm{O}}_3 \\ \mathrm{For}, {\mathrm{H}}_3\stackrel{+1}{\mathrm{P}}{\mathrm{O}}_2\to \stackrel{-3}{\mathrm{P}}{\mathrm{H}}_3 \\ \mathrm{E}=\frac{\mathrm{mol}. \mathrm{wt}.}{\mathrm{change} \mathrm{in} \mathrm{o}.\mathrm{s}}=\frac{\mathrm{M}}{4} \\ \mathrm{For}, {\mathrm{H}}_3\stackrel{+1}{\mathrm{P}}{\mathrm{O}}_2\to {\mathrm{H}}_3\stackrel{+3}{\mathrm{P}}{\mathrm{O}}_3 \\ \mathrm{E}=\frac{\mathrm{M}}{2} \\ \mathrm{Hence}, {\mathrm{E}}_{{\mathrm{H}}_3{\mathrm{PO}}_2}=\frac{\mathrm{M}}{2}+\frac{\mathrm{M}}{4}=\frac{3\mathrm{M}}{4}=\frac{3\times 66}{4}=49.5 \end{gathered}$$