When conc. `H_(2)SO_(4)` was teated with `K_(4)[Fe(CN)_(6)]`, CO gas was evolved. By mistake, somebody used dilute `H_(2)SO_(4)` instead of conc.`H_(2)SO_(4)` then the gas evolved was

Options

A. `CO`
B. `HCN`
C. `N_(2)`
D. `CO_(2)`

Explanation

To solve the problem, we need to analyze the reactions of potassium ferrocyanide (K₄[Fe(CN)₆]) with concentrated and dilute sulfuric acid (H₂SO₄).

### Step 1: Reaction with Concentrated H₂SO₄
When K₄[Fe(CN)₆] reacts with concentrated sulfuric acid, the products formed include potassium sulfate (K₂SO₄), ammonium sulfate ((NH₄)₂SO₄), and carbon monoxide (CO) gas is evolved.

**Balanced Reaction:**
\[ K₄[Fe(CN)₆] + 6 H₂SO₄ \rightarrow 2 K₂SO₄ + 3 (NH₄)₂SO₄ + 6 CO \]

### Step 2: Reaction with Dilute H₂SO₄
Now, if we mistakenly use dilute sulfuric acid instead of concentrated sulfuric acid, the reaction changes. The products formed in this case will include potassium sulfate (K₂SO₄), ferrous sulfate (FeSO₄), and hydrogen cyanide (HCN) gas is evolved.

**Balanced Reaction:**
\[ K₄[Fe(CN)₆] + 6 H₂SO₄ (dilute) \rightarrow 2 K₂SO₄ + FeSO₄ + 6 HCN \]

### Conclusion
From the above analysis, when K₄[Fe(CN)₆] is treated with dilute H₂SO₄, the gas evolved is hydrogen cyanide (HCN).

### Final Answer
The gas evolved when dilute H₂SO₄ is used instead of concentrated H₂SO₄ is **hydrogen cyanide (HCN)**.

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