What is the oxidation state of osmium $(Os)$ in $OsO_4$?

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

What is the oxidation number of $Mn$ in $MnO_{4}^{2-}$ ion?

What is the oxidation state of $Cr$ in potassium dichromate?

Suggest a list of substances where carbon can exhibit oxidation states from $-4$ to $+4$ and nitrogen from $-3$ to $+5$.

When $SO_2$ is passed through an acidic solution of potassium dichromate,chromium sulphate is formed. What is the change in the oxidation state of chromium?