What is the oxidation number of carbon in methanal?

The oxidation state of each oxygen atom in $Na_{2}O_{2}$ is

The oxidation number of carbon in $CH_2Cl_2$ is

Dichromate ion is treated with base,the oxidation number of $Cr$ in the product formed is .... .

What is the oxidation number of oxygen in $OF_2$?

Calculate the oxidation number of sulfur,chromium,and nitrogen in $H_2SO_5$,$Cr_2O_7^{2-}$,and $NO_3^-$. Suggest the structures of these compounds and explain the fallacy in the direct calculation of the oxidation number for $H_2SO_5$.