The coagulation of 200 ml of a colloidal solution of gold is completely prevented by adding `0.50` gm of starch to
it before adding 1 ml of `10%` NaCl solution. Calculate the gold number of starch.

Answer

To calculate the gold number of starch, we follow these steps:

### Step 1: Understand the Concept of Gold Number
The gold number is defined as the minimum weight in milligrams of a protective colloid (in this case, starch) required to prevent the coagulation of 10 mL of a gold colloid when 1 mL of a 10% NaCl solution is added.

### Step 2: Given Information
- Volume of colloidal gold solution = 200 mL
- Amount of starch added = 0.50 g
- Volume of NaCl solution added = 1 mL (10% NaCl)

### Step 3: Calculate the Amount of Starch Required for 10 mL of Gold Solution
Since 0.50 g of starch prevents coagulation for 200 mL of gold solution, we need to find out how much starch is required for 10 mL.

1. **Calculate the amount of starch per mL of gold solution:**
\[ \text{Starch per mL} = \frac{0.50 \text{ g}}{200 \text{ mL}} = 0.0025 \text{ g/mL} \]

2. **Calculate the amount of starch required for 10 mL:**
\[ \text{Starch for 10 mL} = 0.0025 \text{ g/mL} \times 10 \text{ mL} = 0.025 \text{ g} \]

### Step 4: Convert Grams to Milligrams
To express the amount of starch in milligrams:
\[ 0.025 \text{ g} = 0.025 \times 1000 \text{ mg} = 25 \text{ mg} \]

### Step 5: Conclusion
The gold number of starch is therefore 25 mg. This means that 25 mg of starch is required to prevent the coagulation of 10 mL of gold colloid when 1 mL of 10% NaCl is added.

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

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