Question

Given below are two statements : one is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A) : Osmolarity is affected by the volumes of the various solutes in the solution, while osmolality is not.
Reason (R) : The osmolarity is the no. of osmoles per kilogram of the solvent whereas the osmolality is the no. of osmoles per litre of the solution.
In the 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 but (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:

A simple mnemonic to never mix these up:
OsmolaRity = per LitRe (Volume).
OsmolaLity = per kiLogram (Mass).
Clinical labs prefer measuring osmolality because it is temperature-independent!

Answer 1

The Correct Option is C

Step 1: Concept:
This question tests the critical physical chemistry distinction between two closely related concentration metrics used constantly in clinical fluid therapy: Osmolarity and Osmolality.

Step 2: Step-by-step Explanation:

• Analyzing Reason (R): Let's establish the definitions first.
- OsmolaRity (with an 'R') is officially defined as the number of osmoles of solute per Litre of solution (volume).
- OsmolaLity (with an 'L') is defined as the number of osmoles of solute per Kilogram of solvent (mass).
- Look closely at Reason (R): It completely swaps these two definitions! It claims osmolarity is per kg, and osmolality is per litre. Therefore, Reason (R) is fundamentally FALSE.

• Analyzing Assertion (A): Because OsmolaRity relies on the total volume of the solution (litres), it is directly affected by anything that changes volume—including temperature fluctuations, pressure changes, and the physical space (volumes) taken up by large solutes (like proteins or lipids).
- Conversely, OsmolaLity relies strictly on the mass of the solvent (kg). Mass does not expand or contract with temperature, nor is it displaced by the volume of other solutes. Therefore, it remains constant regardless of these factors.
- Therefore, Assertion (A) is a perfectly accurate scientific statement.

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