The oxidation state of nitrogen in $N_3H$ is

The oxidation number of $N$ in $NH_3$ is:

What is the oxidation number of oxygen in $KO_2$?

Oxidation number of osmium $(Os)$ in $OsO_4$ is

In $KO_2$,the nature of oxygen species and the oxidation state of oxygen atom are,respectively

In the chromyl chloride test for the confirmation of $Cl^{-}$ ion,a yellow solution is obtained. Acidification of the solution and the addition of amyl alcohol and $10\% H_2O_2$ turns the organic layer blue,indicating the formation of chromium pentoxide. The oxidation state of chromium in that compound is: