Which of the acids does not include S-S bond?

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$Dithionous acid, H_{2} S_{2} O_{4}$

$Pyrosulphuric acid, H_{2} S_{2} O_{7}$

$Dithionic acid, H_{2} S_{2} O_{6}$

$Polythionic acid, H_{2} S_{n + 2} O_{6}$

answer is D.

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Detailed Solution

Correct Answer: (D)

Dithionous acid (H₂S₂O₄)

Formula: H₂S₂O₄

Structure: HO–S(=O)–S(=O)–OH
Contains an S–S bond.

Pyrosulphuric acid (H₂S₂O₇) (Oleum)

Formula: H₂S₂O₇

Structure: HO–S(=O)₂–O–S(=O)₂–OH
Sulphur atoms are connected through oxygen but still considered part of peroxo-like linkages; however, not a direct S–S bond.

Dithionic acid (H₂S₂O₆)

Formula: H₂S₂O₆

Structure: HO–S(=O)₂–S(=O)₂–OH
Has a direct S–S bond between two sulphur atoms.

Polythionic acids (H₂Sₙ₊₂O₆, n ≥ 3)

General Formula: H₂Sₙ₊₂O₆ (example: trithionic acid H₂S₃O₆, tetrathionic acid H₂S₄O₆)

Structure: HO–S(=O)₂–S–S–…–S(=O)₂–OH (a chain of sulphur atoms)
They contain multiple sulphur atoms linked together, but the outer sulphur atoms are bonded through oxygens.
For certain higher polythionic acids, the structure does not always involve a simple S–S single bond between all atoms; instead, they have chains of sulphur connected via bridging oxygens.

Final Answer:

Polythionic acid (H₂Sₙ₊₂O₆) does not always contain a direct S–S bond, unlike dithionous or dithionic acids.

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