Which of the acids below has the most number of ionizable hydrogen atoms per molecule?

(P)$\begin{array}{r}{\mathrm{H}}_3{\mathrm{PO}}_4\end{array}$(Q) ${\mathrm{H}}_3{\mathrm{PO}}_3$

(R) ${\mathrm{H}}_3{\mathrm{PO}}_2$

The structure of pyrophosphorous acid,($\begin{array}{r}{\mathrm{H}}_3{\mathrm{PO}}_4\end{array}$)and pyrophosphorous acid,(${\mathrm{H}}_3{\mathrm{PO}}_3$) shows that it has two acidic or ionisable hydrogen.

While (${\mathrm{H}}_3{\mathrm{PO}}_2$) is a monobasic acid and also has only one ionisable H, two H-atoms are direcdy attached to phosphorus.

Hence,each one contains the same number of ionizable H atom