On adding one mL of solution of 10% NaCl to 100 mL of gold sol in the presence of 0.25 g of starch, the coagulation is just prevented. The gold number of starch is

Options

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

Explanation

To determine the gold number of starch based on the given information, we can follow these steps:

### Step-by-Step Solution:

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

2. **Convert the Mass of Starch to Milligrams**:
- Since 1 g = 1000 mg, we convert 0.25 g to mg:
\[ 0.25 \, \text{g} = 0.25 \times 1000 \, \text{mg} = 250 \, \text{mg} \]

3. **Calculate the Amount of Starch Required for 10 mL of Gold Sol**:
- The amount of starch (250 mg) is used to prevent coagulation in 100 mL of gold sol. We need to find out how much starch would be required for 10 mL of gold sol.
- Using the ratio:
\[ \text{Amount of starch for 10 mL} = \frac{250 \, \text{mg} \times 10 \, \text{mL}}{100 \, \text{mL}} = 25 \, \text{mg} \]

4. **Calculate the Gold Number**:
- The gold number is defined as the amount of protective colloid (in mg) required to prevent the coagulation of 1 mL of a standard NaCl solution (10%).
- From the previous step, we found that 25 mg of starch is required to prevent coagulation of 1 mL of 10% NaCl solution.
- Therefore, the gold number of starch is:
\[ \text{Gold Number} = 25 \]

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

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