# How can the law of definite proportions be used to differentiate between compounds?

Tell me the ratio of $N : O$ in the following nitrogen oxides: $N O$; $N {O}_{2}$; ${N}_{2} {O}_{5}$?
All of these nitrogen oxides are real species, but manifestly have different elemental proportions. Of course, the law cannot be used to differentiate the radical $N {O}_{2}$, from ${N}_{2} {O}_{4}$. Why not?