Thenard's blue is

Options

A. `CoAl_(2)O_(4)`
B. `Fe_(4)[Fe(CN)_(6)]_(3)`
C. `K_(2)Fe[Fe(CN)_(6)]`
D. `[Cu(NH_(3))_(4)](OH)_(2)`

Answer

**Step-by-Step Solution:**

1. **Identify the Compounds:**
The question provides four compounds:
- (i) CoAl2O4
- (ii) Fe4[Fe(CN)6]3
- (iii) K2[Fe(CN)6]
- (iv) Cu(NH3)4(OH)2

2. **Recall the Definition of Thenard's Blue:**
Thenard's blue is a specific compound known as cobalt aluminate, which has the chemical formula CoAl2O4. It is a blue pigment.

3. **Analyze Each Compound:**
- **CoAl2O4:** This is cobalt aluminate, which matches the definition of Thenard's blue.
- **Fe4[Fe(CN)6]3:** This is not related to cobalt and does not match the definition.
- **K2[Fe(CN)6]:** This is potassium ferrocyanide, which also does not contain cobalt.
- **Cu(NH3)4(OH)2:** This compound contains copper and ammonia, not cobalt.

4. **Conclusion:**
Since CoAl2O4 is the only compound that matches the definition of Thenard's blue, we conclude that:
- **Thenard's blue is CoAl2O4.**

5. **Final Answer:**
The correct option is (i) CoAl2O4.

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