WAEC Chemistry 2012 Paper

Concept:Trioxocarbonate(IV) salts are carbonates. On heating, most metal carbonates leave a solid metal oxide residue, but ammonium carbonate leaves no solid residue.Explanation:Ammonium trioxocarbonate(IV), $(\mathrm{NH}_4)_2\mathrm{CO}_3$, is thermally unstable. On heating, it decomposes completely into gaseous products: $$(\mathrm{NH}_4)_2\mathrm{CO}_3 \xrightarrow{\Delta} 2\mathrm{NH}_3 + \mathrm{CO}_2 + \mathrm{H}_2\mathrm{O}$$ Ammonia gas, carbon(IV) oxide gas, and water vapour are all evolved. Therefore, no solid residue remains. Calcium carbonate, lead(II) carbonate, and zinc carbonate each leave a solid metal oxide residue on heating.Answer:A. ammonium trioxocarbonate (IV)