Consider the following sulphur based oxoacids. $$H_2SO_3, H_2SO_4, H_2S_2O_8$$ and $$H_2S_2O_7$$. Amongst these oxoacids, the number of those with peroxo (O-O) bond is

We are asked to determine how many of the given sulphur-based oxoacids — $$H_2SO_3$$, $$H_2SO_4$$, $$H_2S_2O_8$$, and $$H_2S_2O_7$$ — contain a peroxo (O-O) bond.

A peroxo bond is a direct oxygen-oxygen single bond (-O-O-). Let us examine the structure of each acid:

$$H_2SO_3$$ (Sulphurous acid): The structure has sulphur bonded to three oxygen atoms — two OH groups and one S=O double bond. There is no O-O bond present.

$$H_2SO_4$$ (Sulphuric acid): Sulphur is at the centre, bonded to two OH groups and two S=O double bonds. All oxygen atoms are bonded to sulphur only. There is no O-O bond.

$$H_2S_2O_8$$ (Peroxodisulphuric acid / Marshall's acid): This acid is formed by joining two $$HSO_4$$ units through a peroxo linkage. Its structure is $$HO_3S-O-O-SO_3H$$. The two sulphate-like halves are connected by a direct O-O bond. This is the peroxo bond. So $$H_2S_2O_8$$ contains a peroxo bond.

$$H_2S_2O_7$$ (Pyrosulphuric acid / Oleum acid): This acid is formed by the condensation of two molecules of $$H_2SO_4$$ with loss of water. Its structure is $$HO_3S-O-SO_3H$$, where the two sulphur atoms are connected through a single bridging oxygen atom (S-O-S). There is no O-O bond.

Among the four acids, only $$H_2S_2O_8$$ (peroxodisulphuric acid) contains a peroxo (O-O) bond. The count is therefore 1.

Hence, the correct answer is 1.