The number of reactions that gives `SO_(2)` gas :
(a) `Cu+"conc. "H_(2)SO_(4)rarr`
(b) `Zn +"dil. "H_(2)SO_(4)rarr`
(c ) `S+H_(2)SO_(4)rarrr `
(d) `FeS_(2)+O_(2)rarr`
(e ) `NaHSO_(3)+"dil. "H_(2)SO_(4)rarr`
(f) `H_(2)SO_(4)+PCl_(5)rarr`

Answer

To determine the number of reactions that produce sulfur dioxide (SO₂) gas from the given reactions, we will analyze each reaction step by step.

### Step-by-Step Solution:

1. **Reaction (a): Cu + conc. H₂SO₄ →**
- When copper reacts with concentrated sulfuric acid, it produces copper sulfate (CuSO₄), water (H₂O), and sulfur dioxide (SO₂) gas.
- **Conclusion:** This reaction produces SO₂.

2. **Reaction (b): Zn + dil. H₂SO₄ →**
- Zinc reacts with dilute sulfuric acid to form zinc sulfate (ZnSO₄) and hydrogen gas (H₂).
- **Conclusion:** This reaction does not produce SO₂.

3. **Reaction (c): S + H₂SO₄ →**
- Sulfur reacts with sulfuric acid to produce sulfur dioxide (SO₂) and water (H₂O).
- **Conclusion:** This reaction produces SO₂.

4. **Reaction (d): FeS₂ + O₂ →**
- Iron disulfide (FeS₂) reacts with oxygen to produce iron oxide (Fe₂O₃) and sulfur dioxide (SO₂).
- **Conclusion:** This reaction produces SO₂.

5. **Reaction (e): NaHSO₃ + dil. H₂SO₄ →**
- Sodium hydrogen sulfate reacts with dilute sulfuric acid to produce sodium sulfate (Na₂SO₄), water (H₂O), and sulfur dioxide (SO₂).
- **Conclusion:** This reaction produces SO₂.

6. **Reaction (f): H₂SO₄ + PCl₅ →**
- Sulfuric acid reacts with phosphorus pentachloride to produce sulfur dichloride (SO₂Cl₂), phosphoryl chloride (POCl₃), and hydrochloric acid (HCl).
- **Conclusion:** This reaction does not produce SO₂.

### Summary of Reactions Producing SO₂:
- Reactions that produce SO₂: (a), (c), (d), and (e).
- Total number of reactions producing SO₂: **4**.

### Final Answer:
The number of reactions that give SO₂ gas is **4**.

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