Calculate the oxidation number of sulphur in H₂S₂O₈.

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20. a) Calculate the oxidation number of sulphur in H₂S₂O₈. [1]

Step 1: Assign known oxidation numbers.
In $\text{H}_2\text{S}_2\text{O}_8$, hydrogen has an oxidation number of $+1$, and oxygen usually has $-2$. However, $\text{H}_2\text{S}_2\text{O}_8$ (peroxydisulfuric acid) contains a peroxide linkage $(-\text{O}-\text{O}-)$, where two oxygen atoms have an oxidation number of $-1$. The remaining six oxygen atoms have an oxidation number of $-2