The basicity of \(H_3PO_4\)
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → 3
The basicity of an oxoacid of phosphorus is the number of ionisable H atoms, and only H atoms in P–OH groups ionise. Orthophosphoric acid, \( \ce{H3PO4} \), is \( \ce{O=P(OH)3} \): it has three P–OH groups and no P–H bond, so it ionises in three steps and is tribasic:
\( \displaystyle \ce{H3PO4 + H2O <=> H3O+ + H2PO4-} \)
\( \displaystyle \ce{H2PO4- + H2O <=> H3O+ + HPO4^2-} \)
\( \displaystyle \ce{HPO4^2- + H2O <=> H3O+ + PO4^3-} \)
It therefore forms three series of salts, such as \( \ce{NaH2PO4} \), \( \ce{Na2HPO4} \) and \( \ce{Na3PO4} \).
Note 1: 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 – …
Note 2: The same/similar question has been asked in different years by NTA as per the details given below:
| 2026 | 2025 | 2024 | 2023 | 2022 |
|---|---|---|---|---|
| – | – | – | 20-06 S3 27-05 S1 | – |