The coagulation of `100mL` of a colloidal solution of gold is completely prevented by adding `0.25g` of starch to it before adding `10mL` of `10%NaCl` solution. The gold number of starch is

Options

A. `24`
B. `240`
C. `120`
D. `360`

Explanation

To solve the problem, we need to calculate the gold number of starch based on the information provided. The gold number is defined as the minimum weight in milligrams of a protecting colloid required to prevent the coagulation of 10 mL of a gold solution when a specific amount of an electrolyte (in this case, NaCl) is added.

### Step-by-Step Solution:

1. **Understand the Given Information:**
- Volume of colloidal gold solution = 100 mL
- Mass of starch added = 0.25 g
- Volume of NaCl solution added = 10 mL
- Concentration of NaCl solution = 10%

2. **Determine the Amount of Starch Required for 10 mL of Gold Solution:**
- Since 0.25 g of starch prevents the coagulation of 100 mL of gold solution, we can find out how much starch is required for 10 mL.
- The amount of starch required for 10 mL can be calculated using the ratio:
\[ \text{Starch for 10 mL} = \left(\frac{0.25 \text{ g}}{100 \text{ mL}}\right) \times 10 \text{ mL} \]

3. **Calculate the Amount of Starch:**
\[ \text{Starch for 10 mL} = \frac{0.25 \text{ g}}{100} \times 10 = 0.025 \text{ g} \]

4. **Convert the Amount of Starch to Milligrams:**
- Since the gold number is defined in milligrams, we convert grams to milligrams:
\[ 0.025 \text{ g} = 0.025 \times 1000 \text{ mg} = 25 \text{ mg} \]

5. **Conclusion:**
- The gold number of starch is 25 mg.

### Final Answer:
The gold number of starch is **25 mg**.