On addition of the 1 ml. solution of 10% NaCl to 10ml gold sol in the presence of 0.00399 gm of starch calculate the gold no.

Explanation

To calculate the gold number based on the provided information, follow these steps:

### Step 1: Understand the Definition of Gold Number
The gold number is defined as the minimum amount of protective colloid (in milligrams) required to prevent the color change of a gold sol from red to violet when a specific amount of an electrolyte (in this case, NaCl) is added.

### Step 2: Identify Given Data
- Volume of gold sol = 10 ml
- Volume of NaCl solution = 1 ml
- Concentration of NaCl solution = 10%
- Weight of starch = 0.00399 g

### Step 3: Convert Weight of Starch to Milligrams
To find the gold number, we need the weight of starch in milligrams:
\[ \text{Weight of starch in mg} = 0.00399 \, \text{g} \times 1000 \, \text{mg/g} = 3.99 \, \text{mg} \]

### Step 4: Calculate the Gold Number
The formula for calculating the gold number (G) is:
\[ G = \frac{1000 \times \text{Weight of starch in mg}}{\text{Volume of gold sol in ml}} \]
Substituting the values:
\[ G = \frac{1000 \times 3.99 \, \text{mg}}{10 \, \text{ml}} = \frac{3990}{10} = 399 \, \text{mg/ml} \]

### Step 5: Final Calculation
Since the question asks for the gold number, we can round the value:
\[ G \approx 4 \]

### Conclusion
The gold number is approximately 4.

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