Consider the following reactions:
PCl₃ + H₂O → A + HCl
A + H₂O → B + HCl
The number of ionisable protons present in the product B is _______________.

Solution

Chemistry ma’am · 4 steps

Hydrolysis ends at H₃PO₃, and only the H atoms on oxygen count as ionisable protons.

1. Hydrolysis of PCl3. Water replaces the Cl atoms on phosphorus one by one, and each replacement releases HCl. The two reactions show this stepwise hydrolysis. The final product B is phosphorous acid, H₃PO₃.

   PCl₃ + 3H₂O → H₃PO₃ + 3HCl

2. Real structure. H₃PO₃ is not P(OH)₃. It exists as HPO(OH)₂. Phosphorus carries one P=O, two P-OH groups and one P-H bond.
3. Which H ionises. An O-H hydrogen can leave as H⁺ because the strongly polar O–H bond breaks in water. The P-H bond is almost non-polar, so that hydrogen does not ionise. That makes H₃PO₃ dibasic.
4. So. B has two O-H groups, so the number of ionisable protons is 2.

The common slip: 3. Counting all three H atoms in H₃PO₃ gives 3, but the H bonded directly to P does not ionise.

The idea to keep. In oxoacids of phosphorus, count only the H atoms bonded to oxygen. P–H hydrogens are not acidic: H₃PO₂ is monobasic, H₃PO₃ is dibasic, H₃PO₄ is tribasic.

Answer from the NTA final answer key. Solution by PrepWiser.