Arsenic (III) sulphide forms a sol with a negative charge.
Which of the following ionic substances should be most effective in coagulating the sol?

Options

A. `KCl`
B. `MgCl_(2)`
C. `Al_(2)(SO_(4))_(3)`
D. `Na_(3)PO_(4)`

Explanation

To determine which ionic substance is most effective in coagulating a negatively charged sol of arsenic (III) sulfide (As2S3), we need to consider the charge of the ions present in the options provided and their coagulation power.

### Step-by-Step Solution:

1. **Identify the Charge of the Sol**:
- Arsenic (III) sulfide forms a negatively charged sol. This means that in order to coagulate this sol, we need a positively charged coagulant.

2. **List the Options and Their Cations**:
- The options provided are:
1. KCl (Potassium ion: K⁺)
2. MgCl₂ (Magnesium ion: Mg²⁺)
3. Aluminium Sulfate (Aluminium ion: Al³⁺)
4. Sodium Phosphate (Sodium ion: Na⁺)

3. **Determine the Cation Charges**:
- KCl provides K⁺ (1+ charge)
- MgCl₂ provides Mg²⁺ (2+ charge)
- Aluminium Sulfate provides Al³⁺ (3+ charge)
- Sodium Phosphate provides Na⁺ (1+ charge)

4. **Coagulation Power of Cations**:
- The coagulation power of cations generally increases with the charge of the ion. The order of coagulation power from highest to lowest is:
- Al³⁺ > Mg²⁺ > K⁺ > Na⁺

5. **Flocculation Values**:
- The flocculation values (which indicate the effectiveness of coagulation) for the ions are:
- Sodium ion (Na⁺): 51 mmoles
- Potassium ion (K⁺): 50 mmoles
- Magnesium ion (Mg²⁺): 0.72 mmoles
- Aluminium ion (Al³⁺): 0.093 mmoles
- The lower the flocculation value, the more effective the ion is at coagulating the sol.

6. **Conclusion**:
- Since Al³⁺ has the highest charge and thus the highest coagulation power, Aluminium Sulfate is the most effective in coagulating the negatively charged sol of arsenic (III) sulfide. Therefore, the correct answer is **option 3: Aluminium Sulfate**.