$3 Mg + N _2 \rightarrow Mg _3 N _2$
Orange solid is $\left( NH _4\right)_2 Cr _2 O _7$
Colourless gas is $N _2$
Green residue is $Cr _2 O _3$
$(I)$ $H_5P_3O_{10}$ $(II)$ $H_6 P_4O_{13}$
$(III)$ $H_5P_5O_{15}$ $(IV)$ $H_7P_5O_{16}$
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