`As_2S_3` sol has a negative charge. Capacity to precipitate it is highest in:

Options

A. `AlCl_3`
B. `Na_3PO_4`
C. `CaCl_2`
D. `K_2SO_4`

Answer

To determine which substance has the highest capacity to precipitate the negatively charged `As_2S_3` sol, we can follow these steps:

### Step-by-step Solution:

1. **Identify the Charge of the Sol**:
The question states that `As_2S_3` sol has a negative charge. This means that in order to precipitate or coagulate this sol, we need a positively charged ion.

2. **Understand Coagulation**:
Coagulation is the process where particles clump together to form a precipitate. For a negatively charged sol, positively charged ions are required for this process.

3. **Apply Hardy-Schulze Rule**:
According to the Hardy-Schulze rule, the coagulating power of an ion is directly proportional to its valency. This means that ions with higher valency will have a greater capacity to precipitate the sol.

4. **Examine the Given Options**:
We need to analyze the positive ions present in the options:
- **AlCl3**: Contains Al³⁺ (valency = 3)
- **Na3PO4**: Contains Na⁺ (valency = 1)
- **CaCl2**: Contains Ca²⁺ (valency = 2)
- **K2CO3**: Contains K⁺ (valency = 1)

5. **Compare the Valencies**:
- Al³⁺ has a valency of 3, which is the highest among the options.
- Ca²⁺ has a valency of 2.
- Na⁺ and K⁺ both have a valency of 1.

6. **Conclusion**:
Since Al³⁺ has the highest valency, it will have the greatest coagulating power, and therefore, AlCl3 will have the highest capacity to precipitate the negatively charged `As_2S_3` sol.

### Final Answer:
The capacity to precipitate `As_2S_3` sol is highest in **AlCl3**.