For the coagulation of `500mL` of arsenious sulphide sol, `2mL` of `1MNaCl` is required. What is the flocculation value of `NaCl` ?

Options

A. 3
B. 2
C. 5
D. 4

Explanation

To find the flocculation value of NaCl for the coagulation of arsenious sulphide sol, we can follow these steps:

### Step 1: Understand the Definition
Flocculation value (or coagulation value) is defined as the minimum amount of electrolyte (in millimoles) required to completely coagulate 1 liter of colloidal sol.

### Step 2: Calculate the Amount of NaCl Used
We are given that for 500 mL of arsenious sulphide sol, 2 mL of 1 M NaCl is required.

1. **Calculate the number of moles of NaCl used:**
\[ \text{Moles of NaCl} = \text{Molarity} \times \text{Volume (in liters)} \]
Here, Molarity = 1 M and Volume = 2 mL = 0.002 L.
\[ \text{Moles of NaCl} = 1 \, \text{mol/L} \times 0.002 \, \text{L} = 0.002 \, \text{moles} \]

2. **Convert moles to millimoles:**
\[ 0.002 \, \text{moles} = 0.002 \times 1000 = 2 \, \text{millimoles} \]

### Step 3: Scale Up to 1 Liter
Since the given volume is 500 mL, we need to find out how much NaCl would be required for 1 liter (1000 mL).

1. **Calculate the amount of NaCl required for 1000 mL:**
\[ \text{Flocculation value} = \left(\frac{\text{Amount of NaCl for 500 mL}}{500 \, \text{mL}}\right) \times 1000 \, \text{mL} \]
\[ \text{Flocculation value} = \left(\frac{2 \, \text{millimoles}}{500}\right) \times 1000 = 4 \, \text{millimoles} \]

### Step 4: Conclusion
The flocculation value of NaCl for the coagulation of arsenious sulphide sol is **4 millimoles**.

### Final Answer
The correct option is **4**.

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