In how many of the following reactions, one of the products is obtained as a yellow precipitate ?
`{:(Ba^(2+)+CrO_(4)^(2-),rarr,"Product"),(Hg^(2+)+Co^(2+)+SCN^(Θ),rarr,"Product"),(NH_(4)^(o+)+[PtCl_(6)]^(2-),rarr,"Product"),(Ag^(o+)+CrO_(4)^(2-),rarr,"Product"),(NH_(4)^(o+)+[Co(NO_(2))_(6)]^(3-),rarr,"Product"):}`

Explanation

To determine how many of the given reactions produce a yellow precipitate, we will analyze each reaction one by one.

### Step-by-Step Solution:

1. **Reaction 1: \( \text{Ba}^{2+} + \text{CrO}_4^{2-} \)**
- Barium ions react with chromate ions to form barium chromate (\( \text{BaCrO}_4 \)).
- **Observation**: Barium chromate is a yellow precipitate.
- **Conclusion**: This reaction produces a yellow precipitate.

2. **Reaction 2: \( \text{Hg}^{2+} + \text{Co}^{2+} + \text{SCN}^{-} \)**
- Mercuric ions react with cobalt ions and thiocyanate ions to form a complex.
- **Observation**: The product is a complex that is blue in color.
- **Conclusion**: This reaction does not produce a yellow precipitate.

3. **Reaction 3: \( \text{NH}_4^{+} + [\text{PtCl}_6]^{2-} \)**
- Ammonium ions react with the hexachloroplatinate complex.
- **Observation**: The product \( \text{NH}_4[PtCl_6] \) is a yellow precipitate.
- **Conclusion**: This reaction produces a yellow precipitate.

4. **Reaction 4: \( \text{Ag}^{+} + \text{CrO}_4^{2-} \)**
- Silver ions react with chromate ions to form silver chromate (\( \text{Ag}_2\text{CrO}_4 \)).
- **Observation**: Silver chromate is red in color.
- **Conclusion**: This reaction does not produce a yellow precipitate.

5. **Reaction 5: \( \text{NH}_4^{+} + [\text{Co(NO}_2)_6]^{3-} \)**
- Ammonium ions react with the complex of cobalt and nitrite.
- **Observation**: The product is a complex that is yellow in color.
- **Conclusion**: This reaction produces a yellow precipitate.

### Summary of Results:
- **Reactions producing yellow precipitate**:
- Reaction 1: Yes (Barium chromate)
- Reaction 2: No
- Reaction 3: Yes (Hexachloroplatinate)
- Reaction 4: No
- Reaction 5: Yes (Cobalt nitrite complex)

### Final Count:
- Total reactions producing yellow precipitate: **3 (Reactions 1, 3, and 5)**.

### Answer:
**3**

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