On addition of one mL solution of `10%` NaCl to 10 mL gold sol in the presence fo `0.0250` g of starch, the coagulation is just prevented. Starch has the following gold number

Options

A. `0.025`
B. `0.25`
C. `2.5`
D. `25.`

Explanation

To solve the problem, we need to determine the gold number of starch based on the information provided. The gold number is defined as the minimum amount of a lyophilic colloid (in milligrams) required to prevent the coagulation of a given volume of a gold sol when a specific amount of an electrolyte is added.

### Step-by-step Solution:

1. **Understand the Given Data:**
- Volume of gold sol = 10 mL
- Volume of NaCl solution = 1 mL
- Concentration of NaCl solution = 10%
- Mass of starch = 0.0250 g

2. **Convert the Mass of Starch to Milligrams:**
- Since the gold number is expressed in milligrams, convert the mass of starch from grams to milligrams.
\[ 0.0250 \, \text{g} = 0.0250 \times 1000 \, \text{mg} = 25.0 \, \text{mg} \]

3. **Identify the Electrolyte Added:**
- The electrolyte added is NaCl, which can cause the coagulation of the gold sol. The amount of NaCl solution added is 1 mL of a 10% solution.

4. **Calculate the Amount of NaCl in the Solution:**
- A 10% NaCl solution means there are 10 grams of NaCl in 100 mL of solution. Therefore, in 1 mL of this solution:
\[ \text{Amount of NaCl} = \frac{10 \, \text{g}}{100 \, \text{mL}} \times 1 \, \text{mL} = 0.1 \, \text{g} = 100 \, \text{mg} \]

5. **Determine the Gold Number:**
- The gold number is defined as the minimum amount of starch (in mg) required to prevent the coagulation of 10 mL of gold sol when 1 mL of 10% NaCl is added. Since we have 25.0 mg of starch, this is the gold number.
- Therefore, the gold number of starch is:
\[ \text{Gold Number} = 25.0 \, \text{mg} \]

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

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