How many of the following reagents react with arene diazonium slats to form aryl halides?
i. `HBF_(4)//Delta`
ii. `KHS`
iii. `CuCN//KCN`
iv. `CuBr//HBr`
v. `Cu//HCl`
vi. `KI//Delta`
vii. `CH_(3)CH_(2)OH`
viii. `H_(3)PO_(2)//H_(2)O`

Answer

To determine how many of the given reagents react with arene diazonium salts to form aryl halides, we will analyze each reagent one by one.

### Step-by-Step Solution:

1. **Reagent i: HBF₄ / Δ (Heat)**
- When arene diazonium salt reacts with HBF₄ and heat, it replaces the diazonium group (N₂) with a fluorine atom (F).
- **Product:** Aryl fluoride (Fluorobenzene).
- **Conclusion:** This reagent forms an aryl halide.

2. **Reagent ii: KHS**
- KHS introduces a thiol group (SH) in place of the diazonium group.
- **Product:** Aryl thiol (not an aryl halide).
- **Conclusion:** This reagent does not form an aryl halide.

3. **Reagent iii: CuCN / KCN**
- This reagent replaces the diazonium group with a cyanide group (CN).
- **Product:** Aryl cyanide (not an aryl halide).
- **Conclusion:** This reagent does not form an aryl halide.

4. **Reagent iv: CuBr / HBr**
- In this reaction, the diazonium group is replaced by a bromine atom (Br).
- **Product:** Aryl bromide (Bromobenzene).
- **Conclusion:** This reagent forms an aryl halide.

5. **Reagent v: Cu / HCl**
- This reagent replaces the diazonium group with a chlorine atom (Cl).
- **Product:** Aryl chloride (Chlorobenzene).
- **Conclusion:** This reagent forms an aryl halide.

6. **Reagent vi: KI / Δ (Heat)**
- In this case, the diazonium group is replaced by an iodine atom (I).
- **Product:** Aryl iodide (Iodobenzene).
- **Conclusion:** This reagent forms an aryl halide.

7. **Reagent vii: CH₃CH₂OH (Ethanol)**
- This reagent does not replace the diazonium group with a halide; instead, it leads to the formation of a hydrocarbon.
- **Product:** Benzene (not an aryl halide).
- **Conclusion:** This reagent does not form an aryl halide.

8. **Reagent viii: H₃PO₂ / H₂O**
- This reagent also does not replace the diazonium group with a halide; it leads to the formation of benzene.
- **Product:** Benzene (not an aryl halide).
- **Conclusion:** This reagent does not form an aryl halide.

### Summary of Findings:
- Reagents that form aryl halides:
- HBF₄ (i)
- CuBr / HBr (iv)
- Cu / HCl (v)
- KI (vi)

**Total Count of Reagents that Form Aryl Halides:** 4

### Final Answer:
**4 reagents react with arene diazonium salts to form aryl halides.**

---