Question

Given below are two statements, one is labeled as Assertion (A), and the other one is labeled as Reason (R).
Assertion (A): The molality of a solution in a liquid state changes with temperature.
Reason (R): The volume of a solution changes with a change in temperature.
In light of the above statements, choose the most appropriate answer from the options given below.

• A. Both (A) and (R) are correct and (R) is the correct explanation of (A).
• B. Both (A) and (R) are correct and (R) is NOT the correct explanation of (A).
• C. (A) is correct but (R) is not correct.
• D. (A) is not correct but (R) is correct.

Hint:

To remember concentration behaviors:
- Mass-based terms (molality, mass percentage, mole fraction) do NOT change with temperature.
- Volume-based terms (molarity, normality, volume percentage) DO change with temperature.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Concentration terms in chemistry are classified as temperature-dependent or temperature-independent depending on whether they involve mass or volume measurements.

Step 2: Detailed Explanation:
Let us analyze both statements:
- Assertion (A): "The molality of a solution in a liquid state changes with temperature."
Molality (\(m\)) is defined as the number of moles of solute dissolved per kilogram of solvent:
\[ m = \frac{\text{moles of solute}}{\text{mass of solvent in kg}} \]
Since both the moles of solute and the mass of the solvent are mass-based quantities, they do not change with temperature.
Therefore, the molality of a solution remains constant as temperature changes, making Assertion (A) false.
- Reason (R): "The volume of a solution changes with a change in temperature."
Liquids expand or contract when heated or cooled.
Thus, the volume of a liquid solution is temperature-dependent.
This is why volume-based concentration terms like molarity (\(M\)) and normality (\(N\)) change with temperature, while mass-based terms like molality do not.
Therefore, Reason (R) is true.
Thus, (A) is incorrect but (R) is correct.

Step 3: Final Answer:
(A) is not correct but (R) is correct.