Gold number of starch is 25. How much of it is
required to prevent coagulation of 100 ml of gold sol
adding 1 ml of `10% NaCl` solution?

Options

A. 25 mg
B. 250 mg
C. `2.5 mg`
D. `0.250 mg`

Answer

To solve the problem, we need to determine how much starch is required to prevent the coagulation of 100 ml of gold sol when 1 ml of 10% NaCl solution is added, given that the gold number of starch is 25.

### Step-by-Step Solution:

1. **Understand the Gold Number**: The gold number of a protective colloid (in this case, starch) is defined as the minimum weight (in milligrams) of the protective colloid required to prevent the coagulation of a standard gold sol when a specific amount of an electrolyte (1 ml of 10% NaCl solution) is added.

2. **Identify the Given Values**:
- Gold number of starch = 25 mg
- Volume of gold sol = 100 ml
- Volume of NaCl solution added = 1 ml of 10% NaCl

3. **Determine the Amount of Starch for 10 ml of Gold Sol**: According to the definition of the gold number, 25 mg of starch is required to prevent coagulation of 10 ml of gold sol when 1 ml of 10% NaCl is added.

4. **Calculate the Amount of Starch for 100 ml of Gold Sol**:
- Since 25 mg of starch is required for 10 ml, we can set up a proportion to find out how much is needed for 100 ml.
- The calculation is as follows:

\[ \text{Amount of starch required} = \left(\frac{25 \text{ mg}}{10 \text{ ml}}\right) \times 100 \text{ ml} = \frac{25 \times 100}{10} = 250 \text{ mg} \]

5. **Conclusion**: Therefore, to prevent the coagulation of 100 ml of gold sol when adding 1 ml of 10% NaCl solution, 250 mg of starch is required.

### Final Answer:
**250 mg of starch is required.**

---