Question

Observe the following statements.
Statement - I: The boiling point of 0.1 M urea solution is less than that of 0.1 M KCl solution.
Statement - II: Elevation of boiling point is inversely proportional to molar mass of solute.
The correct answer is

• A. Both statements I and II are correct
• B. Statement I is correct, but statement II is not correct
• C. Statement I is not correct, but statement II is correct
• D. Both statements I and II are not correct

Hint:

Colligative properties are defined by the *number* of particles. When comparing solutions of the same molar concentration, the one with the higher van't Hoff factor (\( i \)) will have the greater effect.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:

Boiling point elevation (ΔTb) is a colligative property, which means it depends on the number of solute particles present in the solution.
Formula:
ΔTb = i × Kb × m
where i is the van't Hoff factor.

Step 2: Detailed Explanation:

Analysis of Statement I:

Urea is a non-electrolyte, so it does not dissociate.
i = 1
Effective concentration = 0.1 × 1 = 0.1

KCl is an electrolyte and dissociates into K+ and Cl-.
i = 2
Effective concentration = 0.1 × 2 = 0.2

Since ΔTb is directly proportional to the effective concentration (i × C), the elevation in boiling point for KCl is greater than that for urea.

Higher elevation means higher boiling point.
So,
B.P.(KCl) > B.P.(Urea)

Therefore,
B.P.(Urea) < B.P.(KCl)

Hence, Statement I is correct.

Analysis of Statement II:

Statement II says:
"Elevation of boiling point is inversely proportional to molar mass of solute."

Colligative properties depend on the number of solute particles, that is, on the number of moles present in the solution.

Although the formula
ΔTb = Kb × (wB × 1000) / (MB × wA)
shows MB in the denominator, this inverse relation is valid only when the mass of solute and solvent are fixed.

As a general statement, boiling point elevation is not simply inversely proportional to molar mass.
It also depends on the van't Hoff factor i, which is very important for electrolytes.

In this question, both solutions are 0.1 M, so molar mass is not the deciding factor here.
The difference in boiling point arises because KCl dissociates, while urea does not.

Hence, Statement II is not correct.

Step 3: Final Answer:

Statement I is correct, but Statement II is not correct.