What is the charge on a ferrous ion?

Ozonolysis of $ClO_2$ produces an oxide of chlorine. The average oxidation state of chlorine in this oxide is . . . . .

State whether the following statements are True or False:
$(1)$ The oxidation number of nitrogen in ammonium chloride $(NH_4Cl)$ is $(-3)$.
$(2)$ The oxidation number of $C$ in methyl chloride $(CH_3Cl)$ is $(-2)$.
$(3)$ In the following reaction,the oxidation number of $Br$ changes from $0$ to $+5$: $Br_2 \to BrO_3^-$.

The oxidation number of $Cr$ in $K_3CrO_8$ is $+5$. How many peroxy linkages are present in this molecule?

What is the correct order of oxidation states of sulfur in the following anions: $SO_3^{2-}$,$S_2O_4^{2-}$,and $S_2O_6^{2-}$?

If $HNO_3$ changes into $N_2O$,the oxidation number of nitrogen is changed by