The coagulation of 10 ml of colloidal solution of gold is completely prevented by addition of 0.25 g of a substance "X" to it before addition of 1 ml of 10% NaCl solution. The gold number of "X" is

Answer

To find the gold number of substance "X," we will 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 just prevent the coagulation of 10 ml of a given gold sol when 1 ml of a 10% NaCl solution is added to it.

### Step 2: Identify the Given Data
- Volume of colloidal gold solution = 10 ml
- Mass of substance "X" added = 0.25 g
- Volume of NaCl solution added = 1 ml
- Concentration of NaCl solution = 10%

### Step 3: Convert the Mass of Substance "X" to Milligrams
Since the gold number is expressed in milligrams, we need to convert grams to milligrams:
\[ 0.25 \text{ g} = 0.25 \times 1000 \text{ mg} = 250 \text{ mg} \]

### Step 4: Calculate the Amount of Substance "X" Required per 10 ml of Gold Sol
The amount of substance "X" required to prevent coagulation when 1 ml of 10% NaCl is added is already given as 250 mg for the 10 ml of colloidal gold solution.

### Step 5: Conclusion
The gold number of substance "X" is 250 mg, as this is the amount required to prevent coagulation of the colloidal gold solution.

### Final Answer
The gold number of "X" is **250 mg**.

---