For preparing 0.1 N solution of a compound from its impure sample, of which the percentage purity is known, the weight of the substance required will be

Options

A. More than the theoretical weight
B. Less than the theoretical weight
C. Same as theoretical weight
D. None of these

Answer

To determine the weight of the impure sample required to prepare a 0.1 N solution of a compound, we need to follow these steps:

### Step 1: Understand Normality
Normality (N) is a measure of concentration equivalent to the number of equivalents of solute per liter of solution. For a 0.1 N solution, this means there are 0.1 equivalents of the solute in 1 liter of solution.

### Step 2: Determine the Equivalent Weight
To calculate the weight of the substance needed, we first need to know the equivalent weight of the compound. The equivalent weight can be calculated using the formula:
\[ \text{Equivalent Weight} = \frac{\text{Molar Mass}}{n} \]
where \( n \) is the number of equivalents per mole of the substance.

### Step 3: Calculate the Required Amount of Pure Compound
Using the equivalent weight, we can calculate the amount of pure compound needed for a 0.1 N solution:
\[ \text{Weight of pure compound required} = \text{Normality} \times \text{Equivalent Weight} \times \text{Volume of solution (in L)} \]
For 1 liter of a 0.1 N solution, this simplifies to:
\[ \text{Weight of pure compound required} = 0.1 \times \text{Equivalent Weight} \]

### Step 4: Adjust for Purity
Since the sample is impure, we need to account for its percentage purity. If the percentage purity of the sample is \( P\% \), then the weight of the impure sample required can be calculated using:
\[ \text{Weight of impure sample} = \frac{\text{Weight of pure compound required}}{P/100} \]
This accounts for the fact that only a portion of the impure sample is the actual compound.

### Step 5: Final Calculation
Putting it all together, the final formula to calculate the weight of the impure sample needed is:
\[ \text{Weight of impure sample} = \frac{0.1 \times \text{Equivalent Weight}}{P/100} \]

### Example Calculation
If the equivalent weight of the compound is 50 g/equiv and the percentage purity of the sample is 80%, the calculation would be:
1. Calculate the weight of pure compound required:
\[ \text{Weight of pure compound required} = 0.1 \times 50 = 5 \text{ g} \]
2. Calculate the weight of the impure sample:
\[ \text{Weight of impure sample} = \frac{5}{80/100} = \frac{5}{0.8} = 6.25 \text{ g} \]

### Conclusion
Thus, to prepare a 0.1 N solution from an impure sample, you would need to weigh out 6.25 g of the impure sample in this example.

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