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.

Identify the correct statement for following reaction.

$$3 \mathrm{Mg}+\mathrm{N}_2 \longrightarrow \mathrm{Mg}_3 \mathrm{~N}_2$$