Equivalent weight of ${\mathrm{H}}_2{\mathrm{PO}}_2$ when it disproportionates into ${\mathrm{PH}}_3 \mathrm{and} {\mathrm{H}}_3{\mathrm{PO}}_3$ is (mol.wt.of ${\mathrm{H}}_3{\mathrm{PO}}_2=\mathrm{M})$

.The disproportionation reaction is 3H₃​PO₂​→PH₃​+2H₃​PO_3​.

n-factor =No. of electrons involved/No. of molecules involved  ​= 4/3​ 

Eq. wt. = mol.wt / n−factor​=M / (4/3)​ 

 The equivalent weight of phosphorus is 3M/4