Consider the following reactions: PCl + H O → A + HCl A + H O → B + HCl The number of ionisable prot — p Block Elements Chemistry Question

💡 Solution & Explanation

**Step 1: Identify product A from the first reaction** PCl₃ + H₂O → A + HCl PCl₃ hydrolyzes to form phosphorous acid (H₃PO₃), which is product A. PCl₃ + 3H₂O → H₃PO₃ + 3HCl **Step 2: Identify product B from the second reaction** A + H₂O → B + HCl H₃PO₃ + H₂O → B + HCl H₃PO₃ (phosphorous acid) further hydrolyzes to form phosphoric acid (H₃PO₄), which is product B. H₃PO₃ + H₂O → H₃PO₄ + HCl **Step 3: Determine the structure of product B** H₃PO₄ (phosphoric acid) has the structure: H-O-PO(OH)₂ **Step 4: Count ionisable protons in H₃PO₄** Ionisable protons are those that can be released in aqueous solution (protons bonded to oxygen atoms in -OH groups). H₃PO₄ has three OH groups, but only **2 protons are truly ionisable**. The third hydrogen is bonded directly to phosphorus (P-H bond) and is not ionisable under normal conditions. Therefore, the answer is **2**.