The boiling point of a solvent is raised by the presence of a solute.

• A. Mole fraction
• B. Mass percent
• C. Molality
• D. Boiling Point Elevation

Answer: (D)

Boiling point elevation is a colligative property: when a nonvolatile solute is dissolved in a solvent, the boiling point of the solvent increases. This happens because the solute lowers the solvent’s vapor pressure; according to Raoult’s law, the vapor pressure of the solution is reduced relative to the pure solvent. To reach the external pressure at which boiling occurs, the solution must be heated to a higher temperature. The amount by which the boiling point rises is proportional to the number of dissolved particles, described by ΔTb = iKb m, where m is the molality and i is the van’t Hoff factor. So the phenomenon is boiling point elevation—the terms mole fraction, mass percent, and molality describe concentrations, but the observed effect in this context is the elevation of the boiling point.