The gas evolved in which of the following reactions forms the iodide of Millon's base on being passed through a solution of `[HgI_(4)]^(2-)` in KOH?

Options

A. `CaSO_(4)` treated with dilute HCl
B. `NH_(4)Cl` boiled with NaOH
C. `ZnS` treated with dilute `H_(2)SO_(4)`
D. `MgCO_(3)` heated alone

Answer

To solve the question regarding which gas evolved forms the iodide of Millon's base when passed through a solution of \([HgI_4]^{2-}\) in KOH, we can follow these steps:

### Step 1: Understand the Reagent
The solution of \([HgI_4]^{2-}\) in KOH is known as Nessler's reagent, which is used to detect the presence of ammonia gas (\(NH_3\)).

**Hint:** Recall that Nessler's reagent is specifically used for detecting ammonia.

### Step 2: Identify the Reaction
We need to identify a reaction that produces ammonia gas. A common reaction that produces ammonia is the reaction of ammonium chloride (\(NH_4Cl\)) with sodium hydroxide (\(NaOH\)).

**Hint:** Look for reactions involving ammonium salts and strong bases, as they often produce ammonia gas.

### Step 3: Write the Reaction Equation
The reaction between ammonium chloride and sodium hydroxide can be written as follows:
\[ NH_4Cl + NaOH \rightarrow NaCl + NH_3 + H_2O \]
This shows that when ammonium chloride is boiled with sodium hydroxide, ammonia gas is produced along with sodium chloride and water.

**Hint:** Make sure to balance the equation correctly to see all products formed.

### Step 4: Reaction with Nessler's Reagent
When the ammonia gas produced from the above reaction is passed through Nessler's reagent, it reacts to form a brown precipitate known as the iodide of Millon's base, which has the formula \(HgO \cdot HgI \cdot NH_2\).

**Hint:** Remember that the formation of a precipitate indicates a successful reaction between the gas and the reagent.

### Step 5: Conclusion
Based on the above steps, we conclude that the gas evolved in the reaction of ammonium chloride with sodium hydroxide is ammonia, which forms the iodide of Millon's base when passed through Nessler's reagent.

**Final Answer:** The correct answer is the reaction of ammonium chloride with sodium hydroxide, which produces ammonia gas.