A compound contains 28% N and 72% of a metal by weight. Three atoms of metal combine with two atoms ofN. Find the atomic weight of metal.

Since, 3 atoms of the metal combine with 2 atoms of nitrogen.

So, M formula ${\mathrm{m}}_3{\mathrm{N}}_2{\mathrm{m}}^{2+}{\mathrm{N}}^3$

At M mass = 14 g  eq. of mass = $\frac{14}{3}g$

28g N combine with = 72g metre 

1g N combine with = $\frac{72}{28}\mathrm{g}\text{ metal }$

$\frac{14}{3}g$N combine with = $\frac{72}{28}\times \frac{14}{3}$

Eq. mass of metal = 12g 

At mass = eq. mass $\times$ valency

$=12\times 2=24g$

Atomic mass = 24g