Write the structure of H₃PO₃ and explain why it is diprotic, not triprotic — JEE Chemistry

Structure of $H_3PO_3$ (phosphorous acid):

$$P is bonded to: 2 OH groups, 1 P=O, 1 P-H (direct P–H bond)$$

Only the two –OH groups can ionise:

$$H_3PO_3 \rightleftharpoons H^+ + H_2PO_3^- \rightleftharpoons 2H^+ + HPO_3^{2-}$$

The direct $P–H$ bond is non-ionisable.

$$K_{a1} \approx 5\times10^{-2},\quad K_{a2} \approx 2\times10^{-7}$$

$$Diprotic because one H is directly bonded to P (not to O)$$