When a crystalline compound X is heated with `K_(2)Cr_(2)O_(7)` and concentrated `H_(2)SO_(4)`, a reddish brown gas A is evolved. On passing A into caustic soda, a yellow solution of B is formed. A yellow precipitate of C is obtained when a solution B is neutralised with acetic acid and then treated with a lead acetate solution. when X is heated with `NaOH`, a colourless gas is evolved which, when passed into a solution of `K_(2)[HgI_(4)]`, gives a reddish brown precipitate of D. ltBrgt Q. Which of the following is the composition of the brown precipitate (D) ?

Options

A. `HgI_(2)`
B. `Hg(NH_(2))I`
C. `HgO`
D. `HgO*Hg(NH_(2))I`

Answer

To solve the given problem step by step, we will analyze the reactions and identify the compounds involved.

### Step 1: Identify Compound X
The question states that a crystalline compound X is heated with `K2Cr2O7` and concentrated `H2SO4`, producing a reddish-brown gas A. The only compound that fits this description is **ammonium chloride (NH4Cl)**. When heated with potassium dichromate and sulfuric acid, it produces chromyl chloride (CrO2Cl2), which is the reddish-brown gas A.

**Hint:** Look for a compound that contains both ammonium ions and a halide ion, which can react with dichromate.

### Step 2: Formation of Gas A
When ammonium chloride (NH4Cl) is heated with `K2Cr2O7` and concentrated `H2SO4`, it produces chromyl chloride (CrO2Cl2), which is the reddish-brown gas A.

**Hint:** Consider the reaction of ammonium salts with strong oxidizing agents in acidic conditions.

### Step 3: Reaction of Gas A with Caustic Soda
Gas A (CrO2Cl2) is passed into caustic soda (NaOH), resulting in the formation of a yellow solution B, which is sodium chromate (Na2CrO4).

**Hint:** Identify the products formed when chromyl chloride reacts with a strong base.

### Step 4: Formation of Yellow Precipitate C
When the yellow solution B (Na2CrO4) is neutralized with acetic acid (CH3COOH) and treated with lead acetate (Pb(CH3COO)2), a yellow precipitate C, which is lead chromate (PbCrO4), is formed.

**Hint:** Think about the reactions that occur when chromate ions are treated with lead ions in an acidic medium.

### Step 5: Reaction of Compound X with NaOH
When compound X (NH4Cl) is heated with NaOH, a colorless gas (ammonia, NH3) is evolved. This gas is then passed into a solution of `K2[HgI4]` (Nessler's reagent), resulting in a reddish-brown precipitate D.

**Hint:** Consider the identification of ammonia using Nessler's reagent and the type of precipitate formed.

### Step 6: Formation of Brown Precipitate D
The reddish-brown precipitate D formed when ammonia reacts with Nessler's reagent is **HgO·HgNH2·I** (iodide of melons base), which confirms the presence of ammonia.

**Hint:** Look for the specific compound formed when ammonia interacts with mercury(II) iodide in the presence of hydroxide ions.

### Conclusion
The composition of the brown precipitate D is **HgO·HgNH2·I**.

**Final Answer:** The composition of the brown precipitate D is **HgO·HgNH2·I**.