Gold metal will not dissolve in either concentrated nitric acid or 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.

We know that gold metal will dissolve in a mixture of concentrated nitric acid (HNO3) and concentrated hydrochloric acid (HCl) to form AuCl4- ions and gaseous NO. So, the unbalanced equation can be represented as: \[ Au + HNO_3 + HCl \rightarrow AuCl_{4}^{-} + NO (g) \]

Now, we need to balance the equation. To do this, we will apply the law of conservation of mass which states that the count of individual atoms must be equal on both sides of the equation. 1. Gold (Au): On both sides of the equation, we have one gold atom, so this component is already balanced. 2. Nitrogen (N): On the left side, we have 1 nitrogen atom and on the right side, we have 1 nitrogen atom, so this component is also balanced. 3. Hydrogen (H): On the left side, there's a total of 1+1 = 2 hydrogen atoms, and on the right side, there's a total of 4 hydrogen atoms. To balance the hydrogen, we need to multiply the HCl by 4: \[ Au + HNO_3 + 4HCl \rightarrow AuCl_{4}^{-} + NO (g) \] 4. Oxygen (O): On the left side, we have a total of 3+ (4×0) = 3 oxygen atoms, and on the right side, we have a total of 2 oxygen atoms. We can balance the oxygen atoms by multiplying the HNO3 by 2: \[ Au + 2HNO_3 + 4HCl \rightarrow AuCl_{4}^{-} + 2NO (g) \] 5. Chlorine (Cl): On the left side, we have 4 chlorine atoms, and on the right side, we also have 4 chlorine atoms. So, the chlorine component is balanced. Finally, the balanced chemical equation for the dissolution of gold in aqua regia is: \[ Au + 2HNO_3 + 4HCl \rightarrow AuCl_{4}^{-} + 2NO (g) \]