The oxidation number of $Cr$ in $K_2Cr_2O_7$ is

From the given list,the number of compounds with $+4$ oxidation state of Sulphur is $ . . . . . . $.
$SO_3, H_2SO_3, SOCl_2, SF_4, BaSO_4, H_2S_2O_7$

The oxidation number of carbon in $CH_2O$ is

$A$ compound of $Xe$ and $F$ is found to have an atomic ratio $Xe : F$ as $0.4 : 2.4$. Find the oxidation number of $Xe$.

The oxidation states of $S$ in $H_2S_2O_7$ are:

The characteristic oxidation number of atoms in free metals is