on addition of 1 ml solution of `10% NaCl` to 10 ml gold sol in the presence of 0.25g of strach, the coagulation is just prevented. Strach has the following gold number

Options

A. `0.025`
B. `0.25`
C. `0.5`
D. `250`

Explanation

To find the gold number of starch in the given problem, we will follow these steps:

### Step-by-Step Solution:

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

2. **Identify the Given Data**:
- Volume of gold sol = 10 ml
- Volume of NaCl solution added = 1 ml
- Concentration of NaCl solution = 10%
- Amount of starch used = 0.25 g

3. **Convert the Amount of Starch to Milligrams**:
Since the gold number is expressed in milligrams, we need to convert the amount of starch from grams to milligrams.
\[ 0.25 \text{ g} = 0.25 \times 1000 \text{ mg} = 250 \text{ mg} \]

4. **Conclusion**:
The gold number of starch is the amount of starch that prevents coagulation, which we calculated to be 250 mg.

### Final Answer:
The gold number of starch is **250**.

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