Question

Aqua regia is a mixture of concentrated HCl and concentrated \(HNO_3\) in the ratio of ____________ respectively.

• A. 1 : 2
• B. 2 : 1
• C. 1 : 3
• D. 3 : 1

Hint:

Remember the composition of Aqua Regia:
- 3 parts HCl (ends with 'l', looks like '3' in some mnemonics).
- 1 part HNO\(_3\) (has '3' in formula, so it takes the '1' part in ratio to balance the '3's).
This simple rule helps prevent swapping the numbers.

Answer 1

The Correct Option is D

Step 1: Understanding the Question:
The question asks for the standard mixing volume ratio of concentrated hydrochloric acid (\(HCl\)) and concentrated nitric acid (\(HNO_3\)) used to prepare the highly corrosive reagent known as Aqua Regia.

Step 2: Key Formula or Approach:
Aqua regia (Latin for "royal water") is prepared by mixing concentrated hydrochloric acid and concentrated nitric acid in a specific volumetric ratio.

Step 3: Detailed Explanation:

• Aqua regia is a freshly prepared, highly corrosive, fuming liquid.

• It is made by mixing concentrated Hydrochloric acid (\(HCl\)) and concentrated Nitric acid (\(HNO_3\)) in a volume ratio of 3:1 respectively:
\[ 3\text{ parts conc. } HCl + 1\text{ part conc. } HNO_3 \] - This reagent is one of the few chemical liquids that can dissolve noble metals like gold (\(Au\)) and platinum (\(Pt\)), which are highly resistant to individual strong acids.

• The mixture is powerful because of the synergistic reaction between the two acids. Nitric acid acts as a strong oxidizing agent, oxidizing hydrochloric acid to produce nascent chlorine and nitrosyl chloride:
\[ HNO_3 + 3HCl \to NOCl + 2H_2O + 2[Cl]\text{ (nascent chlorine)} \] - The highly reactive nascent chlorine attacks the noble metals to form soluble chloride complexes, such as tetrachloroaurate (\([AuCl_4]^-\)), which dissolves the metal.

• Therefore, the volume ratio of \(HCl\) to \(HNO_3\) is 3:1.


Step 4: Final Answer:
The ratio of concentrated HCl to concentrated \(HNO_3\) is 3:1.