The dark blue colour of the solution formed when excess of ammonia is added to a solution of copper (II) sulphate is due to the presence of the ion

Options

A. `[Cu(OH)_(4)(H_(2)O)_(2)]^(2-)`
B. `[Cu(H_(2)O)_(6)]^(2+)`
C. `[Cu(NH_(3))_(2)(H_(2)O)_(4)]^(2+)`
D. `[Cu(NH_(3))_(4)(H_(2)O)_(2)]^(2+)`

Answer

To solve the question regarding the dark blue color of the solution formed when excess ammonia is added to a solution of copper (II) sulfate, we can follow these steps:

### Step 1: Understand the Initial Reaction
When copper (II) sulfate (CuSO₄) is dissolved in water, it dissociates into copper ions (Cu²⁺) and sulfate ions (SO₄²⁻). The copper ions are initially surrounded by water molecules, forming a complex ion.

**Hint:** Remember that copper (II) sulfate in solution exists as Cu²⁺ and SO₄²⁻ ions.

### Step 2: Adding Ammonia
When ammonia (NH₃) is added to the copper (II) sulfate solution, it can interact with the copper ions. Initially, a small amount of ammonia may form a light blue precipitate of copper (II) hydroxide (Cu(OH)₂) if the amount is limited.

**Hint:** Consider what happens to the copper ions when ammonia is introduced.

### Step 3: Formation of the Complex Ion
Upon adding excess ammonia, the ammonia molecules can displace the water molecules coordinated to the copper ions. This leads to the formation of a new complex ion, specifically tetraamminecopper(II) ion, which is represented as [Cu(NH₃)₄]²⁺. The reaction can be summarized as follows:

\[ \text{Cu}^{2+} + 4 \text{NH}_3 \rightarrow [\text{Cu(NH}_3\text{)}_4]^{2+} \]

**Hint:** Focus on how the ammonia molecules replace the water molecules around the copper ion.

### Step 4: Color Change
The formation of the tetraamminecopper(II) complex is responsible for the dark blue color of the solution. This complex has distinct optical properties that give it a characteristic deep blue color.

**Hint:** The color change indicates the formation of a specific complex ion.

### Conclusion
The dark blue color of the solution formed when excess ammonia is added to a solution of copper (II) sulfate is due to the presence of the tetraamminecopper(II) ion, [Cu(NH₃)₄]²⁺.

**Final Answer:** The dark blue color is due to the presence of the ion [Cu(NH₃)₄]²⁺.