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

What is the oxidation number of underlined species in $\underline{P}F_6^{-}$ and $\underline{V}_2 O_7^{4-}$ ions respectively?

The oxidation number and covalency of sulphur in the sulphur molecule $S_8$ are respectively

$A$ compound contains atoms $X$,$Y$,and $Z$. The oxidation number of $X$ is $+2$,$Y$ is $+5$ and $Z$ is $-2$. Therefore,a possible formula of the compound is

What is the oxidation number of carbon in $CH_2O$?

$X + Y \rightarrow$ oleum. The sum of oxidation states of the central atom in $X$ and $Y$ is: