On addition of one mL solution of 10% NaCl to 10 mL gold sol in presence of 0.025 g of starch, the coagulation is just prevented. The gold number of starch is:

Question

Accepted Answer

The gold number is a measure of the protective power of a colloid, such as starch, in preventing the coagulation of a gold sol. It is defined as the minimum amount of the protective colloid (in milligrams) required to prevent the coagulation of 10 mL of a gold sol when 1 mL of a 10% NaCl solution is added.

To determine why Option 1 (25) is the correct answer, let's analyze the situation:

1. **Understanding the Gold Number**: The gold number is expressed in milligrams. In this scenario, 0.025 grams of starch is used to prevent coagulation. Converting grams to milligrams, we have:
   $$
   0.025 \, \text{g} = 25 \, \text{mg}
   $$
   This means that 25 mg of starch is required to prevent the coagulation of 10 mL of gold sol when 1 mL of 10% NaCl is added.

2. **Justification for Option 1**: Since the gold number is defined as the amount in milligrams, and we have calculated that 25 mg of starch is needed, Option 1 (25) is indeed the correct answer. This aligns with the definition of the gold number, confirming that 25 mg is the minimum amount required to prevent coagulation under the given conditions.

3. **Explanation for Incorrectness of Other Options**:
   - **Option 2 (2.5)**: This option suggests that only 2.5 mg of starch is needed, which is incorrect because the calculation shows that 25 mg is required.
   - **Option 3 (0.25)**: This option implies that 0.25 mg of starch is sufficient, which is far less than the calculated 25 mg, making it incorrect.
   - **Option 4 (0.025)**: This option indicates that only 0.025 mg is needed, which is incorrect as it is significantly lower than the required 25 mg.

In conclusion, the correct answer is Option 1 (25), as it accurately represents the amount of starch needed to prevent coagulation in the given scenario. The other options are incorrect because they do not match the calculated requirement of 25 mg.