Which oxoacid of phosphorous has the highest number of oxygen atoms present in its chemical formula?

Solution

Chemistry ma’am · 3 steps

Write each acid's formula and count the O atoms. Two-P acids win, and pyrophosphoric has the most.

1. Pyro means condensed. A pyro acid forms when two molecules of the parent acid lose one water molecule between them. So 2H₃PO₄ - H₂O gives H₄P₂O₇, and 2H₃PO₃ - H₂O gives H₄P₂O₅.
2. Hypophosphoric acid. Hypophosphoric acid, H₄P₂O₆, also has two P atoms, but they are joined directly by a P–P bond, not by a P–O–P bridge. That is why it has one oxygen fewer than pyrophosphoric acid.

   Acid	Formula	Oxidation state of P	O atoms
   Pyrophosphorous acid	H₄P₂O₅	+3	5
   Hypophosphoric acid	H₄P₂O₆	+4	6
   Phosphoric acid	H₃PO₄	+5	4
   Pyrophosphoric acid	H₄P₂O₇	+5	7

3. So. Pyrophosphoric acid, H₄P₂O₇, has 7 oxygen atoms, the most of the four. The answer is (d).

The idea to keep. Build pyro acids yourself: take two parent acid molecules and remove one H₂O. Two P atoms with P at +5 is what gives the largest oxygen count.

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