The sum of oxidation states of all atoms in $Cr_{2}O_{7}^{2-}$ ion is

The oxidation states of $P$ atom in $POCl_3$,$H_3PO_3$ and $H_4P_2O_6$,respectively are

The oxidation number of an element in a compound is evaluated on the basis of certain rules. Which of the following rules is not correct?
$1$. The oxidation number of hydrogen is always $+1$.
$2$. The algebraic sum of all the oxidation numbers of all elements in a compound is zero.
$3$. An element in the free or the uncombined state bears oxidation number zero.
$4$. In all its compounds,the oxidation number of fluorine is $-1$.

The oxidation number of sulphur in $S_8$ molecule is

$2$ mole of $N_2H_4$ loses $16$ mole of electrons and is converted to a new compound $X$. Assuming that all of the $N$ atoms appear in the new compound,what is the oxidation state of $N$ in $X$?

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