The coagulation of `10cm^(3)` of gold sol by `1 ml 10% NaCl` solution is completely prevented by addition of `0.025g` of starch to it. The gold number of starch is

Options

A. 0.025
B. 0.25
C. 2.5
D. 250

Answer

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

### Step 1: Understand the Definition of Gold Number
The gold number is defined as the weight of a protective colloid (in milligrams) required to prevent the coagulation of 10 cm³ of a gold sol when a specific amount of an electrolyte (in this case, NaCl) is added.

### Step 2: Identify the Given Data
- Volume of gold sol = 10 cm³
- Amount of NaCl solution = 1 ml of 10% NaCl
- Weight of starch added = 0.025 g

### Step 3: Convert the Weight of Starch to Milligrams
Since the gold number is expressed in milligrams, we need to convert the weight of starch from grams to milligrams:
\[ 0.025 \text{ g} = 0.025 \times 1000 \text{ mg} = 25 \text{ mg} \]

### Step 4: Calculate the Gold Number
According to the definition, the gold number of starch is equal to the weight of starch required to prevent coagulation of 10 cm³ of gold sol. Since we have added 25 mg of starch to prevent coagulation, the gold number of starch is:
\[ \text{Gold Number} = 25 \text{ mg} \]

### Conclusion
The gold number of starch is 25 mg.

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