Question

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

Answer 1

The impurity doesn't contribute to the normality/ molarity of the solution. So, we need to take more samples than the theoretical weight so that the required weight of the compound in the sample can be obtained.

Complete answer:
Let’s understand the situation with an example that if we are taking a compound, with molecular weight 40g having 10% impurity. If we want to prepare 1000 ml, 0.1N solution of that compound. The weight of sample we need to take can be calculated from the following formula:
$N = \dfrac{W}{{MW}} \times \dfrac{{1000}}{V}$
Where N= normality of solution
W= Weight of sample taken in grams
MW= molecular weight of sample compound in grams
V= volume of sample to be prepared in ml.
To calculate the weight of sample needed to prepare the solution of required concentration convert the equation in following way:
$\Rightarrow W = \dfrac{N}{{1000}} \times MW \times V$
Now solve this for given sample where N=0.1N, MW=40, V=100ml
$\Rightarrow W=\dfrac{{0.1 \times 40 \times 1000}}{{1000}}$
$=4 g$
We need 4 g of sample but 10% is impurity, then how much sample we need to take to get 4g of pure compound?
Answer is $\dfrac{{100}}{{90}} \times 4$=4.44g.
We need 4.44 g of sample if we want 4 g of pure compound to form the required solution.

So the option (B) is correct.

Note:
Just use general mathematics to be sure about the answer otherwise the theory is so clear to predict the answer. Impurity doesn't contribute to concentration so sample weight should be taken more.