Question

Assertion: For very dilute solutions, the strength of solution in $ \dfrac{W}{W}% $ and in $ \dfrac{W}{V}% $ have nearly equal value.
Reason: For a very dilute solution, the mass of solution becomes almost equal to the mass of solvent.
(A) Both Assertion and Reason are correct and Reason is the correct explanation for Assertion.
(B) Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion
(C) Assertion is incorrect but Reason is correct
(D) Assertion is correct but Reason is incorrect

Answer 1

Hint: Here, we need to check the truth value of assertion and reason as individual statements, and if both the statements are proved correct, then we have to check if the reason statement correctly explains the assertion statement. Here, by very dilute solutions, we need to understand that the amount of solute is very less as compared to the amount of solvent.

Complete answer:
Let us consider the assertion statement “For very dilute solutions, the strength of solution in $ \dfrac{W}{W}% $ and in $ \dfrac{W}{V}% $ have nearly equal value”
 $ \dfrac{W}{W}% $ known as the weight per weight concentration, can be defined as the percentage fraction of the amount of solute (in grams) and the amount of solution (in grams).
 $ \dfrac{W}{V}% $ known as the weight per volume concentration, can be defined as the percentage fraction of the amount of solute (in grams) and the amount of solute (in millilitres)
Now, as the solution is very dilute, the amount of solute in the numerator can be ignored. Thus, for both fractions to be equal, the weight of solution and the volume of solution should be the same.
But, this is not possible for liquids other than water. Hence, due to the density, weight is not equal to the volume.
Thus the assertion statement is incorrect.
Now, the mass of solution is the total sum of the mass of the solute and the mass of the solvent.
For very dilute solutions, the amount of solute is very less as compared to the amount of solvent. Hence, in addition to solute to solvent, the amount of solvent, which is a solution now, almost remains unchanged.
Thus, the mass of the solution is almost equal to the mass of the solvent.
Hence, the reason statement is correct.
Hence, the correct answer is Option $ (C) $ .

Note:
The point to remember is the assertion-reason type of questions, is that we need to check if by using reason statements we can correctly explain the assertion statement, given both statements are correct. Also, if the solution is very dilute, the amount of solute in the weight fractions $ \dfrac{W}{W}% $ , $ \dfrac{W}{V}% $ , $ \dfrac{V}{V}% $ can be ignored. Hence, the value for $ \dfrac{W}{V}% $ and $ \dfrac{V}{V}% $ is nearly equal.