Reaction of thionyl chloride with white phosphorus forms a compound [A], which on hydrolysis gives [B], a dibasic acid. [A] and [B] are respectively :

Solution

Chemistry ma’am · 4 steps

The word dibasic gives it away: that is H₃PO₃, so [A] must be the chloride that hydrolyses to it.

1. Form [A]. Thionyl chloride chlorinates white phosphorus to phosphorus trichloride. This is the standard NCERT preparation of PCl₃.

   P₄ + 8SOCl₂ → 4PCl₃ + 4SO₂ + 2S₂Cl₂

2. Hydrolyse [A]. Water replaces each Cl on P: PCl₃ + 3H₂O → H₃PO₃ + 3HCl. So [B] is phosphorous acid, H₃PO₃.
3. Why dibasic. In H₃PO₃ the phosphorus carries one P=O, two P-OH and one P-H. Only the two O–H hydrogens are ionisable, so it is dibasic. H₃PO₄ has three P–OH groups and is tribasic.
4. So. [A] is PCl₃ and [B] is H₃PO₃, option (d).

The idea to keep. Basicity of a phosphorus oxoacid counts only the P–OH groups. H atoms bonded directly to P are not ionisable, so H₃PO₃ is dibasic and H₃PO₂ is monobasic.

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