An aqueous solution of an unknown inorganic salt is treated with...

The reagent mixture of $\text{NH}_4\text{Cl}$, $\text{NH}_4\text{OH}$, and $(\text{NH}_4)_2\text{CO}_3$ is the group reagent for Group V cations in systematic qualitative analysis.

The Group V cations include $\text{Ba}^{2+}$, $\text{Sr}^{2+}$, and $\text{Ca}^{2+}$. They precipitate as their respective insoluble carbonates (e.g., $\text{SrCO}_3$).

$\text{Mg}^{2+}$ belongs to Group VI and does not precipitate here because the presence of $\text{NH}_4\text{Cl}$ suppresses the ionization of $(\text{NH}_4)_2\text{CO}_3$, keeping the carbonate ion concentration too low to exceed the solubility product of $\text{MgCO}_3$.

$\text{Zn}^{2+}$ (Group IV)$and \text{Cu}^{2+}$ (Group II) precipitate with different group reagents. Therefore, the cation present is $\text{Sr}^{2+}$.

Answer: $\text{Sr}^{2+}$