Colligative properties — If you prepare separate solutions by dissolving equimolar amounts of different non-electrolyte solutes in the same mass of a given solvent, what will be the elevation in boiling point for each solution (at the same pressure)?
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AThe same elevation in boiling point for all solutions
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BDifferent elevations in boiling point for different solutes
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CElevations in boiling point proportional to molecular weights of the solutes
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DNo elevation at all because the solutes are different
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EElevation inversely proportional to the solvent’s Kb
Answer
Correct Answer: The same elevation in boiling point for all solutions
Explanation
Introduction:Boiling-point elevation is a classic colligative property. Colligative properties depend only on the number of solute particles present, not their chemical identity (assuming ideal dilute solutions and no association/dissociation).
Given Data / Assumptions:
- Equimolar amounts (same moles) of different solutes are dissolved.
- The same mass of the same solvent is used for each solution.
- Solutes are non-electrolytes (van’t Hoff factor i ≈ 1) and solutions are dilute.
- Pressure is the same for all cases.
Concept / Approach:Boiling-point elevation is given by ΔT_b = i * K_b * m, where m is molality and K_b is the ebullioscopic constant of the solvent. With equal moles of solute in the same solvent mass, m is identical for all solutions. With i ≈ 1 and the same K_b (same solvent), ΔT_b is the same for each solution, regardless of solute identity.
Step-by-Step Solution:Set m = n_solute / kg_solvent; with same n_solute and same kg_solvent → same m for all.Use ΔT_b = i * K_b * m; with i ≈ 1 and same K_b → equal ΔT_b.Conclude that each solution exhibits the same boiling-point elevation.
Verification / Alternative check:If a solute dissociates (i > 1) or associates (i < 1), ΔT_b would differ. The problem context specifies different non-electrolytes to keep i ≈ 1.
Why Other Options Are Wrong:
- (b) Conflicts with the definition of a colligative property under the stated assumptions.
- (c) ΔT_b does not scale with molecular weight; it scales with particle count (molality).
- (d) Boiling point does elevate for nonvolatile solutes.
- (e) K_b is a constant for the solvent and is the same in each case, not a variable linking differences here.
Common Pitfalls:Forgetting van’t Hoff factors for electrolytes; mixing up molarity and molality in boiling-point elevation problems.
Final Answer:The same elevation in boiling point for all solutions