Consider the following reaction :
A + alkali → B (Major Product)
If B is an oxoacid of phosphorus with no P-H bond, then A is :

Solution

Chemistry ma’am · 3 steps

Match each starting material to the oxoacid it gives, then count the P–H bonds in that acid.

1. Which hydrogens count. In an oxoacid of phosphorus, a hydrogen on oxygen (P–OH) is acidic, but a hydrogen bonded directly to P (P–H) is not. You need the starting material whose acid has every hydrogen on oxygen.

   A (treated with alkali)	Oxoacid formed	P–H bonds
   White P₄	H₃PO₂, hypophosphorous acid (with PH₃)	2
   Red P₄	H₄P₂O₆, hypophosphoric acid	0
   Oxide P₂O₃	H₃PO₃, phosphorous acid	1
   Acid H₃PO₃	Phosphite salt, which keeps the H₃PO₃ framework	1

2. Hypophosphoric acid. H₄P₂O₆ is (HO)₂P(O)-P(O)(OH)₂. The two P atoms, each in the +4 state, are joined by a P–P bond. All four hydrogens sit on oxygen, so there is no P–H bond.
3. So. B is hypophosphoric acid. It comes from red P₄ and alkali, which is option (b).

The idea to keep. Learn the pairings: white P₄ with alkali gives H₃PO₂ (two P–H bonds). Red P₄ with alkali gives H₄P₂O₆, which has no P–H bond and a P–P bond instead.

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