The oxidation number of $S$ in $S^{2-}$ is:

Assign the oxidation number to the underlined element in each of the following species:
$1$. $NaH_{2}\underline{P}O_{4}$
$2$. $NaH\underline{S}O_{4}$
$3$. $H_{4}\underline{P_{2}}O_{7}$

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

What is the oxidation number of sulfur in $Na_2S_4O_6$?

The oxidation states of $S$ in Peroxodisulphuric acid $(H_2S_2O_8)$ and Sodium tetrathionate $(Na_2S_4O_6)$ are respectively:

What are the oxidation numbers of nitrogen in $NH_4NO_3$?