The oxidation state of sulphur in $H_2S_2O_7$ is

When $KMnO_4$ is reduced with oxalic acid in acidic solution,the oxidation number of $Mn$ changes from

What is the oxidation state of oxygen in $O_2^{2-}$ ion?

The oxidation states of $S$ in peroxodisulphuric acid $(H_{2}S_{2}O_{8})$ and sodium tetrathionate $(Na_{2}S_{4}O_{6})$ are respectively:

What are the oxidation numbers of the underlined elements in each of the following and how do you rationalise your results?
$(a)$ $K\underline {I_3}$
$(b)$ $H_2\underline {S_4}O_6$
$(c)$ $\underline {Fe_3}O_4$
$(d)$ $\underline {C}H_3\underline {C}H_2OH$
$(e)$ $\underline {C}H_3\underline {C}OOH$

The oxidation state of sulphur in $SO_4^{2-}$ is