Arrange the following oxoacids of phosphorus in the correct decreasin…

The correct answer is Option (1) → B > D > A > C

The oxoacids of phosphorus that contain P–H bonds are good reducing agents, and the more P–H bonds (and the lower the oxidation state of P), the stronger the reducing power. Acids with no P–H bond are weaker reducing agents, and the acid with P in its highest state (+5) has no reducing power.

• (B) \( \ce{H3PO2} \) — first. 3 + x − 4 = 0, so P is +1; it has two P–H bonds, the most of the four, so it is the strongest reducing agent.

• (D) \( \ce{H3PO3} \) — second. 3 + x − 6 = 0, so P is +3; it has one P–H bond.

• (A) \( \ce{H4P2O6} \) — third. 4 + 2x − 12 = 0, so P is +4; it has no P–H bond, but P is still below +5 and can be oxidised a little further.

• (C) \( \ce{H4P2O7} \) — last. 4 + 2x − 14 = 0, so P is +5, its highest state; with no P–H bond it has no reducing power.

Shortcut: Only Options (1) and (2) start with \( \ce{H3PO2} \) (B); only Option (1) ends with \( \ce{H4P2O7} \) (C, P in +5).

Note: This question is from the pre-rationalised NCERT book (2022-23). The topic is not there in the latest NCERT book (2026-27), and it is also out of the CUET 2026 syllabus.

NCERT (Pre-rationalised edition, 2022-23):
Oxoacids of Phosphorus – “… The acids which contain P–H bond have strong reducing properties. Thus, hypophosphorous acid is a good reducing agent as it contains two P–H bonds and reduces, for example, AgNO₃ to metallic silver.
4 AgNO₃ + 2H₂O + H₃PO₂ → 4Ag + 4HNO₃ + H₃PO₄”