Question

Read the following assertion and reason and choose the correct alternative.
Assertion: Elevation in boiling point for two isotonic solutions is same.
Reason: Boiling point depends upon the concentration of the solute.

• A. Both assertion and reason are true and the reason is the correct explanation of assertion
• B. Both assertion and reason are true but the reason is not the correct explanation of assertion
• C. Assertion is correct but reason is wrong
• D. Assertion is wrong but reason is correct

Hint:

Isotonic means equal osmotic pressure, hence equal concentration; all colligative properties (including \( \Delta T_b = K_b m \)) depend on concentration.

Answer 1

The Correct Option is A

Step 1: Isotonic solutions are defined as solutions that have the same osmotic pressure at a given temperature. From the osmotic pressure relation \( \pi = i\,C\,R\,T \), equal osmotic pressure at the same temperature means the two solutions have the same effective molar concentration \( iC \).
Step 2: Elevation in boiling point is a colligative property given by \( \Delta T_b = i\,K_b\,m \), i.e. it depends on the concentration of solute particles in the solution.
Step 3: Since the two isotonic solutions (in the same solvent) have the same solute concentration, they produce the same elevation in boiling point. Hence the assertion is true.
Step 4: The reason correctly states that the elevation in boiling point depends on the concentration of the solute, and this is exactly why isotonic solutions show the same elevation. So the reason is true and is the correct explanation of the assertion.
Conclusion: Option (i) is correct. Options (iii) and (iv) are wrong because both statements are true, and option (ii) is wrong because the reason does explain the assertion.
\[\boxed{\text{Option (i)}}\]