Gold metal will not dissolve in either concentrated nitric acid on concentrated hydrochloric acid. It will dissolve, however, in aqua regia, a mixture of the two concentrated acids. The products of the reaction are the \(\mathrm{AuCl}_{4}^{-}\) ion and gaseous NO. Write a balanced equation for the dissolution of gold in aqua regia.

First, write an unbalanced equation for the reaction using the given information. The reactants are gold (Au), nitric acid (HNO₃), and hydrochloric acid (HCl). The products are \(\mathrm{AuCl}_{4}^{-}\) ion and nitrogen monoxide (NO). Au + HNO₃ + HCl → \(\mathrm{AuCl}_{4}^{-}\) + NO

In the given reaction, there is a charge imbalance: the product side has a negative charge, while the reactant side has no charge. We need to find a species that can balance the charges. Since the chloride ions come from hydrochloric acid, we can add the counter ion, which is a hydrogen ion (H⁺), as a product: Au + HNO₃ + HCl → \(\mathrm{AuCl}_{4}^{-}\) + NO + H⁺

Now, balance the atoms in the reaction, one element at a time: 1. Gold (Au) is already balanced. 2. To balance the nitrogen (N) atoms, place a coefficient of 4 in front of HNO₃, and a coefficient of 4 in front of the NO: Au + 4HNO₃ + HCl → \(\mathrm{AuCl}_{4}^{-}\) + 4NO + H⁺ 3. To balance the hydrogen (H) atoms, place a coefficient of 4 in front of HCl: Au + 4HNO₃ + 4HCl → \(\mathrm{AuCl}_{4}^{-}\) + 4NO + H⁺ 4. To balance the chlorine (Cl) atoms, place a coefficient of 4 in front of HCl and a coefficient of 1 in front of H⁺: Au + 4HNO₃ + 4HCl → \(\mathrm{AuCl}_{4}^{-}\) + 4NO + 4H⁺ 5. Finally, combine the 4H⁺ ions with the \(\mathrm{AuCl}_{4}^{-}\) ion to form tetrachloroauric acid (HAuCl₄): Au + 4HNO₃ + 4HCl → HAuCl₄ + 4NO

The final balanced equation for the dissolution of gold in aqua regia is: Au + 4HNO₃ + 4HCl → HAuCl₄ + 4NO