The acid in which O–O bond is present is
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → \(H_2S_2O_8\)
\( \ce{H2S2O8} \) is peroxodisulphuric acid (Marshall’s acid). Its two \( \ce{SO3H} \) groups are joined through a peroxo bridge, –O–O–:
\( \ce{HO-S(O2)-O-O-S(O2)-OH} \)
The oxidation state of S is still +6; the two extra oxygens over \( \ce{H2S2O7} \) sit in the peroxide link with oxidation state −1.
Why the other options are incorrect
Option (1) \(H_2S_2O_3\) — Incorrect. Thiosulphuric acid has an S=S (S–S) unit in place of one oxygen, not an O–O bond.
Option (2) \(H_2S_2O_6\) — Incorrect. Dithionic acid has a direct S–S bond between the two \( \ce{SO3H} \) groups.
Option (4) \(H_2S_4O_6\) — Incorrect. Tetrathionic acid has a chain of S–S–S–S bonds; no O–O bond.
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 Sulphur
