Boiling-point elevation (colligative property): Compared with the pure solvent, the boiling point of a solution containing a nonvolatile solute is
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ALess
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BMore
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CThe same
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DEither more or less, depending on the solvent
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EUndefined unless pressure is specified
Answer
Correct Answer: More
Explanation
Introduction / Context:Solutions containing a nonvolatile solute display colligative effects such as boiling-point elevation and freezing-point depression. These depend on solute particle number rather than chemical identity, provided the solute does not dissociate or associate unusually. Boiling-point elevation is routinely used in chemical engineering (e.g., evaporators) and physical chemistry.
Given Data / Assumptions:
- Nonvolatile solute; solvent is volatile.
- Ambient pressure is fixed (e.g., 1 bar) when comparing boiling points.
- Solution is ideal or sufficiently dilute to apply colligative behavior.
Concept / Approach:Adding a nonvolatile solute lowers the solvent’s vapor pressure at a given temperature (Raoult’s law), so to reach the external pressure required for boiling, the solution must be heated to a higher temperature. The elevation ΔTb = Kb * m * i (where Kb is ebullioscopic constant, m is molality, and i is van’t Hoff factor) quantifies the effect.
Step-by-Step Solution:
At fixed pressure, pure solvent boils when Pvap,solvent(Tb,solvent) = Pext.In a solution, Pvap,solvent is reduced by x_solvent; hence at the same T it is below Pext.To compensate, temperature must increase until Pvap,solution(Tb,solution) = Pext.Therefore, Tb,solution > Tb,solvent (boiling-point elevation).Verification / Alternative check:Measured ebullioscopic constants (e.g., water Kb ≈ 0.512 K·kg/mol) predict positive ΔTb for real solutions, confirming the qualitative result.
Why Other Options Are Wrong:
- Less / Same: contradict the basic colligative behavior.
- 'Either more or less': for nonvolatile solutes, the direction is consistently elevation.
- 'Undefined unless pressure is specified': pressure is assumed fixed during comparison.
Common Pitfalls:Confusing nonvolatile with volatile solutes; ignoring dissociation (i) which changes magnitude but not direction for electrolytes.
Final Answer:More