On addition of 1 ml of solution of 10% NaCl to 100ml corporate gold sol in presence of 0.25g of starch, the coagulation is just prevented. The gold number of starch is

Options

A. `0.025`
B. `0.25`
C. 2.5
D. 25

Answer

To determine the gold number of starch based on the given data, we can follow these steps:

### Step 1: Understand the Concept of Gold Number
The gold number is defined as the minimum amount of protective colloid (in grams) required to prevent the coagulation of a given volume of gold sol (in mL) when a specific amount of an electrolyte (like NaCl) is added.

### Step 2: Identify the Given Data
From the question, we have:
- Volume of gold sol = 100 mL
- Amount of NaCl solution added = 1 mL of 10% NaCl
- Mass of starch = 0.25 g

### Step 3: Calculate the Amount of NaCl
The concentration of the NaCl solution is given as 10%. This means that in 100 mL of solution, there are 10 g of NaCl. Therefore, in 1 mL of this solution:
\[ \text{Mass of NaCl} = \frac{10 \text{ g}}{100 \text{ mL}} \times 1 \text{ mL} = 0.1 \text{ g} \]

### Step 4: Apply the Formula for Gold Number
The formula for calculating the gold number (G) is:
\[ G = \frac{\text{Mass of starch (g)}}{\text{Volume of gold sol (mL)}} \]
In this case, the volume of gold sol is 100 mL, and the mass of starch is 0.25 g.

### Step 5: Substitute the Values into the Formula
Substituting the values into the formula gives:
\[ G = \frac{0.25 \text{ g}}{100 \text{ mL}} = 0.0025 \text{ g/mL} \]

### Step 6: Calculate the Gold Number
Since the gold number is typically expressed in terms of the volume of gold sol that can be stabilized per gram of protective colloid, we can rearrange our calculation:
\[ \text{Gold Number} = \frac{\text{Volume of gold sol (mL)}}{\text{Mass of starch (g)}} \]
Thus:
\[ \text{Gold Number} = \frac{100 \text{ mL}}{0.25 \text{ g}} = 400 \]

### Final Answer
The gold number of starch is **400**.

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