Oxoacids of Phosphorus JEE 2021: Official Multi-Correct Solution

What was the 2021 JEE Advanced multi-correct question on oxoacids of phosphorus?

The 2021 JEE Advanced multi-correct question required evaluation of four statements on H3PO3 and H3PO4, with more than one option correct. The statements covered thermal disproportionation of H3PO3 yielding H3PO4 and PH3, the relative ability of H3PO3 versus H3PO4 to function as reducing agent, the claim that H3PO3 is monobasic, and the statement that the hydrogen in the P-H bond of H3PO3 does not ionize in water. The official correct choices are A, B and D.

You can expect to evaluate any statement on H3PO3 or H3PO4 properties correctly after this article because it follows the official solution's emphasis on the single HPO(OH)2 structure that settles oxidation state, disproportionation, reducing character, basicity and ionizability at once.

How does the structure of H3PO3 govern its oxidation state and properties?

H3PO3 is represented as HPO(OH)2. This shows exactly two O-H bonds and one direct P-H bond.

The oxidation state of phosphorus follows

yielding x = +3. H3PO4 has phosphorus at +5. This single structural fact decides every subsequent property.

What does the official solution say option by option for the 2021 JEE Advanced oxoacids question?

The official solution starts from the structure HPO(OH)2 with two O-H bonds and one P-H bond. Oxidation state of P in H3PO3 comes from

yielding +3.

On heating, H3PO3 disproportionates as

$4{\mathrm{H}}_3\mathrm{P}{\mathrm{O}}_3\to 3{\mathrm{H}}_3\mathrm{P}{\mathrm{O}}_4+\mathrm{P}{\mathrm{H}}_3$.

In H3PO4 phosphorus has oxidation state +5; in PH3 it is -3. Phosphorus at +3 is both oxidised to +5 and reduced to -3, so statement A is correct.

H3PO3 acts as a reducing agent because phosphorus at +3 can be oxidised to +5. H3PO4 already has phosphorus at its highest common oxidation state of +5, so it cannot act as a reducing agent under normal conditions. Statement B is correct.

Basicity depends solely on the number of ionizable O-H hydrogens. H3PO3 has two such bonds, making it dibasic; the claim that it is monobasic is false, so statement C is incorrect.

The hydrogen directly bonded to phosphorus in the P-H bond is non-ionizable in water; only O-H hydrogens ionize. This confirms statement D. The correct options are therefore A, B and D.

This walkthrough matches the verified official solution exactly. Search every JEE Advanced paper from 2007 onwards in the past-paper archive to see every variant with its worked solution.

Why do students select the monobasic claim for H3PO3?

Students treat basicity as equal to the total number of hydrogen atoms appearing in the molecular formula instead of counting only those attached to oxygen. They see H3 in H3PO3 and classify it as monobasic.

The structure HPO(OH)2 shows exactly two ionizable O-H groups. Failure to separate the non-acidic P-H bond from the acidic O-H bonds produces the incorrect classification of H3PO3 as monobasic.

How can I test the same logic on nearby p-block concepts?

Apply the identical method to these fresh questions.

State the basicity of H3PO2 and H4P2O7 using only the number of OH groups. H3PO2 has one OH and two P-H bonds, so it is monobasic. H4P2O7 has four OH groups, so it is tetrabasic. Count O-H linkages; ignore the H subscript.

Does hypophosphorous acid (H3PO2) undergo disproportionation on heating and what products form? Yes. The balanced reaction is

$2{\mathrm{H}}_3\mathrm{P}{\mathrm{O}}_2\to {\mathrm{H}}_3\mathrm{P}{\mathrm{O}}_4+\mathrm{P}{\mathrm{H}}_3$.

Phosphorus moves from +1 both upward to +5 and downward to -3, exactly parallel to the H3PO3 case.

Multi-correct comparison of reducing nature and maximum oxidation state across H3PO2, H3PO3 and H3PO4. Correct statements are that reducing power follows H3PO2 > H3PO3 > H3PO4 because initial oxidation states are +1, +3 and +5 respectively, while all three can reach a maximum oxidation state of +5. Solve by writing structures first.

Work these before your next mock. Each reinforces the HPO(OH)2 diagnostic tool from the official solution.

What checklist should I follow for oxoacids of phosphorus in the final 30 days?

Always begin by writing the correct condensed structure showing distinct P-H and P-OH linkages. Never start from the molecular formula alone.

Calculate oxidation number of P first to decide possible redox behavior. For H3PO3 the value +3 immediately signals both oxidation to +5 and reduction to -3 are feasible.

Count only O-H bonds for basicity; never use the subscript of H in the formula. Two O-H means dibasic, one O-H means monobasic.

Remember the specific 4:3:1 stoichiometry of the H3PO3 disproportionation

$4{\mathrm{H}}_3\mathrm{P}{\mathrm{O}}_3\to 3{\mathrm{H}}_3\mathrm{P}{\mathrm{O}}_4+\mathrm{P}{\mathrm{H}}_3$.

Reproduce it from memory in under 30 seconds.

Drill this sequence on every p-block acid question until the structure appears automatically. That habit turns medium-difficulty statements into guaranteed marks.

Next step: photograph a doubt on JEEnius and photograph any question you are stuck on and get a step-by-step solution, with a free-body diagram when the question needs one (20 free).

Related on JEEnius: Limit, Continuity and Differentiability JEE 2018: Official Solution.