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, the products are copper(II) sulfate (CuSO₄), sulfur dioxide (SO₂), and water (H₂O).
- **Products:** CuSO₄ + SO₂ + H₂O
- **SO₂ produced:** Yes

2. **Reaction (b): Zn + dil. H₂SO₄ →**
- Zinc reacts with dilute sulfuric acid to produce zinc sulfate (ZnSO₄) and hydrogen sulfide (H₂S), but no sulfur dioxide is formed.
- **Products:** ZnSO₄ + H₂S
- **SO₂ produced:** No

3. **Reaction (c): S + H₂SO₄ →**
- Sulfur reacts with sulfuric acid to produce sulfur dioxide (SO₂) and water (H₂O).
- **Products:** 3SO₂ + 2H₂O
- **SO₂ produced:** Yes (3 moles of SO₂)

4. **Reaction (d): FeS₂ + O₂ →**
- Iron(II) sulfide reacts with oxygen to produce iron(III) oxide (Fe₂O₃) and sulfur dioxide (SO₂).
- **Products:** Fe₂O₃ + SO₂
- **SO₂ produced:** Yes

5. **Reaction (e): NaHSO₃ + dil. H₂SO₄ →**
- Sodium bisulfite reacts with dilute sulfuric acid to produce sodium sulfate (Na₂SO₄), sulfur dioxide (SO₂), and water (H₂O).
- **Products:** Na₂SO₄ + SO₂ + H₂O
- **SO₂ produced:** Yes

6. **Reaction (f): H₂SO₄ + PCl₅ →**
- Sulfuric acid reacts with phosphorus pentachloride to produce phosphorus oxychloride (POCl₃), chlorosulfonic acid (HSO₃Cl), and hydrochloric acid (HCl). No sulfur dioxide is produced in this reaction.
- **Products:** POCl₃ + HSO₃Cl + HCl
- **SO₂ produced:** No

### Summary of SO₂ Production:
- **Reactions producing SO₂:**
- (a) Yes
- (b) No
- (c) Yes (3 moles)
- (d) Yes
- (e) Yes
- (f) No

### Total Reactions Producing SO₂:
- Total = 4 reactions produce SO₂ (from reactions a, c, d, and e).

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

---