Question

The ratio of HCl and HNO$_3$ in aqua regia is

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

Hint:

Aqua regia must be prepared fresh because it loses reactivity quickly as chlorine escapes.

Answer 1

The Correct Option is D

Step 1: What is Aqua Regia? 
Aqua regia (Latin for “royal water”) is a highly corrosive mixture of concentrated hydrochloric acid (HCl) and concentrated nitric acid (HNO$_3$). It is called “royal water” because it can dissolve noble metals like gold and platinum, which neither acid alone can dissolve. 
Step 2: Ratio of Mixing: 
The standard composition of aqua regia is: \[ \text{HCl : HNO}_3 = 3 : 1 \] That is, three parts of concentrated HCl mixed with one part of concentrated HNO$_3$. 
Step 3: Chemistry Behind: 
HNO$_3$ acts as a strong oxidizing agent, producing nascent chlorine. 
Reaction: \[ HNO_3 + 3HCl \rightarrow NOCl + Cl_2 + 2H_2O \] 
Here, chlorine gas and nitrosyl chloride (NOCl) are formed, which help dissolve noble metals. 
Step 4: Final Answer: 
The ratio of HCl and HNO$_3$ in aqua regia is 3 : 1.