If the molecular weight of H₃PO₃ is M, its equivalent weight will be

Equivalent Weight of H₃​PO₃​ = $\frac{\mathrm{Molecular} \mathrm{weight} \mathrm{of} {\mathrm{H}}_3{\mathrm{PO}}_3\left(\mathrm{M}\right)}{\mathrm{Basicity}}$​

Since, H₃​PO₃​ is a dibasic acid i.e. it has two hydrogen ions to donate to a base in acid-base reaction,

${\mathrm{H}}_3{\mathrm{PO}}_3\to 2{\mathrm{H}}^{+} + {\mathrm{HPO}}_3^{-}$

So, equivalent weight of H₃​PO₃​ = $\frac{\mathrm{M}}{2}$

Since, it has a basicity of 2