What is the oxidation number of Mn in $\mathrm{MnO}_4^{-}$?

What is the oxidation state of phosphorus in phosphate ion?

What is change in oxidation number of nitrogen when $\mathrm{NO}_3^{-}$is converted to $\mathrm{NH}_4^{+}$ion?

Calculate the oxidation number of Cr in $\mathrm{CrO}_4^{2-}$ ion and $\mathrm{K}_2 \mathrm{Cr}_2 \mathrm{O}_7$ respectively.