What is the ratio of Concentrated Hydrochloric acid (HCl) is to Conc. Nitric acid ($HNO_{3}$) respectively in Aqua Regia?

Question

Accepted Answer

The correct option is (1):

Aqua Regia is a fuming yellow or red liquid, a highly corrosive mixture of acids, and is notable for its ability to dissolve noble metals like gold and platinum, which cannot be dissolved by a single acid. The term "Aqua Regia" is Latin for "royal water," reflecting its ability to dissolve these "royal" metals. This unique property stems from the synergistic action of its components.

Aqua Regia is prepared by mixing concentrated hydrochloric acid (HCl) and concentrated nitric acid ($\mathrm{HNO_3}$) in a specific molar ratio. The standard and most effective ratio, by volume, of concentrated $\mathrm{HCl}$ to concentrated $\mathrm{HNO_3}$ is $\mathrm{3:1}$.

Here's how it works:  * Nitric acid acts as an oxidizing agent, oxidizing gold to gold(III) ions ($\mathrm{Au^{3+}}$).
    $\mathrm{Au(s) + 3NO\_3^-(aq) + 6H^+(aq) \rightarrow Au^{3+}(aq) + 3NO\_2(g) + 3H\_2O(l)}$
  * Hydrochloric acid then reacts with the gold(III) ions to form tetrachloroaurate(III) anions ($\mathrm{AuCl\_4^-}$). This reaction is crucial because it removes the gold(III) ions from the solution, shifting the equilibrium of the first reaction forward and allowing more gold to be oxidized.
    $\mathrm{Au^{3+}(aq) + 4Cl^-(aq) \rightarrow AuCl\_4^-(aq)}$The combined effect allows for the dissolution of gold.

The incorrect option is (2): A ratio of 1:4 (HCl to $\mathrm{HNO_3}$) is not the correct proportion for Aqua Regia and would not be as effective in dissolving noble metals. The higher proportion of nitric acid would primarily act as an oxidizing agent without sufficient chloride ions to complex the metal ions, thus limiting the dissolution process.

The incorrect option is (3): A ratio of 2:1 (HCl to $\mathrm{HNO_3}$) is not the correct proportion for Aqua Regia. While it contains both acids, this ratio deviates from the optimal 3:1 mixture that provides the best balance for both oxidation and complexation, reducing its effectiveness.

The incorrect option is (4): A ratio of 1:2 (HCl to $\mathrm{HNO_3}$) is not the correct proportion for Aqua Regia. This ratio has too much nitric acid relative to hydrochloric acid, which would not provide enough chloride ions to effectively form soluble metal complexes after oxidation, thus hindering the dissolution of noble metals.