Urea reacts with water to form A, which decomposes to form B. B, when passed through $\mathrm{Cu}{}^{2+}(\mathrm{aq})$, gives a deep-blue coloured solution C. What is the formula of C?

Solution & Explanation

\textbf{Answer:} (D) $[\mathrm{Cu}(\mathrm{NH}{}_3){}_4]{}^{2+}$. \textbf{Solution:} Urea undergoes hydrolysis with water to give ammonium carbamate / carbonate (A), which decomposes to release ammonia gas, $\mathrm{NH}{}_3$ (B): $\mathrm{NH}{}_2\mathrm{CONH}{}_2+\mathrm{H}{}_2\mathrm{O}\to 2\mathrm{NH}{}_3+\mathrm{CO}{}_2$. When excess $\mathrm{NH}{}_3$ is passed through an aqueous $\mathrm{Cu}{}^{2+}$ solution, the ammonia molecules act as ligands and coordinate to the copper(II) centre to form the deep-blue tetraamminecopper(II) complex ion: $\mathrm{Cu}{}^{2+}+4\mathrm{NH}{}_3\to [\mathrm{Cu}(\mathrm{NH}{}_3){}_4]{}^{2+}$. The characteristic deep-blue colour identifies C as the coordination ion $[\mathrm{Cu}(\mathrm{NH}{}_3){}_4]{}^{2+}$, option (D).