During preparation of arenediazonium salts the excess of nitrous acid, if any, is removed by adding

Options

A. Aqueous `Na_2CO_3`
B. Aqueous NaOH
C. Aq.`NH_2CONH_2`
D. `NH_4OH`

Answer

To solve the question regarding the removal of excess nitrous acid during the preparation of arenediazonium salts, we can follow these steps:

### Step 1: Understanding the Reaction
Arenediazonium salts are prepared by treating an aromatic amine with nitrous acid (HNO2). However, sometimes excess nitrous acid remains after the reaction.

**Hint:** Recall the reaction of aromatic amines with nitrous acid to form arenediazonium salts.

### Step 2: Identifying the Problem
The presence of excess nitrous acid can lead to unwanted side reactions or instability in the reaction mixture. Therefore, it is essential to remove any excess nitrous acid.

**Hint:** Consider why it is important to remove excess reagents in chemical reactions.

### Step 3: Using Urea to Remove Excess Nitrous Acid
To remove the excess nitrous acid, we can add urea (NH2CO NH2). Urea reacts with nitrous acid to form an unstable intermediate compound.

**Hint:** Think about how urea might interact with nitrous acid based on its functional groups.

### Step 4: Reaction of Urea with Nitrous Acid
When urea reacts with nitrous acid, it forms an unstable compound that quickly decomposes into carbon dioxide (CO2), nitrogen gas (N2), and water (H2O). This reaction effectively removes the excess nitrous acid from the mixture.

**Hint:** What products are formed when urea reacts with nitrous acid, and why are they significant?

### Step 5: Conclusion
By adding urea, the excess nitrous acid is converted into harmless gases (N2 and CO2) and water, which can easily escape from the reaction mixture without affecting the formation of arenediazonium salts.

**Hint:** Summarize the benefits of using urea in this context.

### Final Answer
The excess of nitrous acid during the preparation of arenediazonium salts is removed by adding **aqueous urea (NH2CO NH2)**.