Oxygen has an oxidation state of $+2$ in which of the following compounds?

The oxidation states of $H$,$N$,and $C$ in $HNC$ are respectively:

Calculate the oxidation number of $Cr$ in $CrO_4^{2-}$ ion and $K_2Cr_2O_7$ respectively.

Amongst the following,identify the species with an atom in $+6$ oxidation state.

The oxidation number of carbon in graphite is ....... .

In the reaction of potassium dichromate,potassium chloride and sulfuric acid (conc.),the oxidation state of the chromium in the product is $(+)$ . . . . . . .