Oxidation number of $S$ in $S_2Cl_2$ is

What is the oxidation number of $S$ in $H_2S_2O_8$?

What is the change in oxidation number of $Cr$ in the following redox reaction?
$3H_2O_2(aq) + Cr_2O_7^{2-}(aq) + 8H^+(aq) \rightarrow 3O_2(g) + 2Cr^{3+}(aq) + 7H_2O(l)$

The oxidation number of $Fe$ and $S$ in iron pyrites are

From the given list,the number of compounds with $+4$ oxidation state of Sulphur is $ . . . . . . $.
$SO_3, H_2SO_3, SOCl_2, SF_4, BaSO_4, H_2S_2O_7$

What is the change in the oxidation number of nitrogen when $NO_3^{-}$ is converted to $NH_4^{+}$ ion?