Arrange the following in increasing order of their pH values :
(A) p-Nitrophenol
(B) m-Cresol
(C) m-Nitrophenol
(D) Phenol
Choose the correct answer from the options given below :
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → (A) < (C) < (D) < (B)
The stronger the acid, the lower the pH of its solution, so the increasing order of pH is the decreasing order of acidic strength. The acidity of a phenol depends on how well its phenoxide ion is stabilised. Two effects decide it:
Electron withdrawing –NO2 group: increases acidity. At the para (or ortho) position it acts by both −R and −I effects, since the negative charge can move onto the nitro group. At the meta position only the weaker −I effect works. So p-nitro > m-nitro in acidity.
Electron donating –CH3 group: by its +I effect it pushes electrons into the ring, destabilises the phenoxide ion and makes the phenol less acidic than phenol.
Matching the given compounds:
(A) p-Nitrophenol: −R and −I effects, the strongest acid, lowest pH.
(C) m-Nitrophenol: −I effect only.
(D) Phenol: no substituent.
(B) m-Cresol: electron donating \( \ce{CH3} \), the weakest acid, highest pH.
Shortcut: p-Nitrophenol (A) has the lowest pH and m-cresol (B) the highest. Only Option (2) starts with (A) and ends with (B).
NCERT (2026-27 edition):
Acidity of Phenols – …
Note: The same/similar question has been asked in different years by NTA as per the details given below:
| 2026 | 2025 | 2024 | 2023 | 2022 |
|---|---|---|---|---|
| 07-06 S2 | – | 29-05 S1 | – | – |