If $HNO_3$ changes into $N_2O$,the oxidation number of nitrogen is changed by

Assign the oxidation number to the underlined element in each of the following species:
$1$. $K_2\underline{Mn}O_4$
$2$. $Ca\underline{O}_2$
$3$. $Na\underline{B}H_4$
$4$. $KAl(\underline{S}O_4)_2 \cdot 12H_2O$

Oxidation state of nitrogen in nitric oxide is

In which of the following oxoacids of phosphorus is the oxidation state of phosphorus the minimum?

What is the oxidation number of carbon in oxalic acid?

The oxidation state of iodine in $I_3^{-}$ is: