For the following oxo acids, choose the increasing order of their acidity:
A. \(HClO < HClO_2 < HClO_4 < HClO_3\)
B. \(HClO_4 < HClO_2 < HClO_3 < HClO\)
C. \(HClO_3 < HClO_4 < HClO_2 < HClO\)
D. \(HClO < HClO_2 < HClO_3 < HClO_4\)
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → D
All four are oxoacids of chlorine: HOCl (Cl +1), HOClO (+3), \( \ce{HOClO2} \) (+5) and \( \ce{HOClO3} \) (+7). As the number of oxygen atoms on chlorine increases, the acid becomes stronger, for two reasons:
the oxidation state of Cl rises, and the extra electronegative O atoms pull electron density away from the O–H bond, so \( \ce{H+} \) is released more easily;
the negative charge of the anion left behind is spread over more oxygen atoms, so the anion is more stable: \( \ce{ClO-} \) < \( \ce{ClO2-} \) < \( \ce{ClO3-} \) < \( \ce{ClO4-} \).
So hypochlorous acid (HClO) is the weakest and perchloric acid (HClO4) the strongest, with HClO2 and HClO3 in between in that order. Sequence D lists them in this increasing order.
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.