Question

Read the following assertion and reason and choose the correct alternative.
Assertion: Osmotic pressure is a colligative property.
Reason: Osmotic pressure is directly proportional to molarity at a given temperature.

• 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:

Use \( \pi = CRT \); at constant T, osmotic pressure tracks molar concentration of particles, which is the very meaning of a colligative property.

Answer 1

The Correct Option is A

Step 1: A colligative property is one that depends only on the number of solute particles present and not on their chemical nature. Osmotic pressure is measured by \( \pi = CRT \), where \( C \) is the molar concentration of the solute particles. Since it depends on the number of particles, osmotic pressure is a colligative property, so the assertion is true.
Step 2: From the van't Hoff relation \( \pi = CRT = \dfrac{n}{V}RT \). At a fixed temperature \( T \), the term \( RT \) is constant, so \( \pi \) is directly proportional to the molar concentration (molarity). Hence the reason is also true.
Step 3: The reason states exactly the mathematical form that makes osmotic pressure depend on concentration of particles, which is precisely why it behaves as a colligative property. Therefore the reason is the correct explanation of the assertion.
Conclusion: Option (i) is correct.
Why others are wrong: (ii) is wrong because the reason does explain the assertion; (iii) and (iv) are wrong because both statements are individually true.