Oxidation number of sulphur in $H_2{S}_2{O}_8$ is $+7$.
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1. 0

The correct option is A 0
Let $x$ be the oxidation number of sulphur in peroxodisulphuric acid or Marshall's acid $H_2{S}_2{O}_8$.
Its structure has been shown at the bottom.
Since the overall charge on the complex is $0$, the sum of oxidation states of all elements in it should be equal to $0$.
We can see that it has a peroxy linkage.
Therefore, $x+2(-2)+2(-1)=0$
or, $x=+6$
Thus, the oxidation number of sulphur in peroxodisulphuric acid or Marshall's acid $H_2{S}_2{O}_8$ is $+6$.