On reacting copper with dilute nitric acid gives a colourless gas (A). The gas (A) on reacting with `Fe^(2+)` ion in concentrated sulphuric acid and water forms a brown coloured complex. The EAN of Iron in the brown coloured complex is : [Given: Atomic No.: Fe = 26]

Options

A. 26
B. 37
C. 25
D. 36

Explanation

To solve the problem step by step, we will follow the reactions and calculations as described in the video transcript.

### Step 1: Identify the gas produced from the reaction of copper with dilute nitric acid.
When copper (Cu) reacts with dilute nitric acid (HNO3), it produces a colorless gas, which is nitric oxide (NO). The balanced reaction can be represented as:

\[ 3 \text{Cu} + 8 \text{HNO}_3 \rightarrow 3 \text{Cu(NO}_3\text{)}_2 + 2 \text{NO} + 4 \text{H}_2\text{O} \]

**Hint:** Remember that dilute nitric acid produces nitric oxide (NO) when reacting with metals like copper.

### Step 2: Understand the reaction of the gas (NO) with Fe²⁺ ions.
The colorless gas (NO) reacts with Fe²⁺ ions in concentrated sulfuric acid and water to form a brown-colored complex known as the nitrosyl complex. The reaction can be represented as:

\[ \text{Fe}^{2+} + 5 \text{H}_2\text{O} + \text{NO} \rightarrow \text{[Fe(H}_2\text{O)}_5\text{NO]}^{2+} \]

**Hint:** The brown ring test is a classic method to identify the formation of this nitrosyl complex.

### Step 3: Determine the oxidation state of iron in the complex.
Let the oxidation state of iron (Fe) in the complex \([Fe(H_2O)_5NO]^{2+}\) be \(x\). The contribution from the water molecules (5) is 0, and from the nitrosyl group (NO) is +1. The overall charge of the complex is +2. Thus, we can set up the equation:

\[ x + 0 + 1 = +2 \]

Solving for \(x\):

\[ x = +2 - 1 = +1 \]

**Hint:** The oxidation state can be found by balancing the charges in the complex.

### Step 4: Calculate the Effective Atomic Number (EAN) of iron in the complex.
The formula for the Effective Atomic Number (EAN) is:

\[ \text{EAN} = Z - n + 2m \]

Where:
- \(Z\) = Atomic number of iron (Fe) = 26
- \(n\) = Number of electrons lost = 1 (since Fe is in +1 oxidation state)
- \(m\) = Coordination number (number of ligands) = 6 (5 water + 1 nitrosyl)

Substituting the values:

\[ \text{EAN} = 26 - 1 + 2 \times 6 \]

Calculating:

\[ \text{EAN} = 26 - 1 + 12 = 37 \]

### Final Answer:
The Effective Atomic Number (EAN) of iron in the brown-colored complex is **37**.

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