$$PCl_5 + H_2O \to A + HCl$$
$$A + H_2O \to B + HCl$$
Find number of ionisable hydrogen in B

We need to find the number of ionisable hydrogens in the final product B.

$$PCl_5 + H_2O \to POCl_3 + 2HCl$$

Product A is phosphorus oxychloride ($$POCl_3$$).

$$POCl_3 + 3H_2O \to H_3PO_4 + 3HCl$$

Product B is phosphoric acid ($$H_3PO_4$$).

Phosphoric acid ($$H_3PO_4$$) is a triprotic acid with 3 ionisable hydrogen atoms, all bonded to oxygen atoms as O-H groups:

$$H_3PO_4 \rightleftharpoons H^+ + H_2PO_4^-$$

$$H_2PO_4^- \rightleftharpoons H^+ + HPO_4^{2-}$$

$$HPO_4^{2-} \rightleftharpoons H^+ + PO_4^{3-}$$

The number of ionisable hydrogens in B is 3.