Sulphur in $+3$ oxidation state is present in

What is the change in oxidation number of selenium in the following redox reaction?
$SeO_{3(aq)}^{2-} + Cl_{2(g)} + 2OH^- \longrightarrow SeO_{4(aq)}^{2-} + 2Cl_{(aq)}^- + H_2O$

In ferrous ammonium sulphate,the oxidation number of $Fe$ is:

What is the oxidation number of sulfur in $H_2SO_5$?

Oxidation number of potassium in $K_2O$,$K_2O_2$ and $KO_2$,respectively,is

The oxidation state of sulphur in $H_2S_2O_7$ is