Ebullioscopic Constant
Definition and meaning of Ebullioscopic Constant in chemistry.
The ebullioscopic constant (Kb) is the proportionality constant that relates the molality of a dissolved solute to the elevation of a solvent's boiling point that the solute causes.
In more detail
Boiling point elevation is a colligative property, so it depends only on the number of dissolved solute particles per kilogram of solvent, not their identity. The relationship is expressed as ΔTb = Kb·m·i, where m is molality and i is the van't Hoff factor accounting for dissociation. Kb is a fixed property of the solvent, determined by its molar mass, normal boiling point, and molar enthalpy of vaporization (Kb = RTb²M/1000ΔHvap). Because Kb is known precisely for common solvents, ebullioscopy (boiling point elevation measurement) can be used to determine the molar mass of an unknown nonvolatile solute.
Key facts
| Symbol | Kb (or Ke) |
|---|---|
| Typical units | °C·kg/mol (equivalently K·kg/mol) |
| Value for water | 0.512 °C·kg/mol |
| Field | Physical Chemistry |
Water has Kb = 0.512 °C·kg/mol. Dissolving 1 mole of glucose (a nonelectrolyte, i = 1) in 1 kg of water raises the boiling point from 100.000 °C to 100.512 °C.
Frequently asked questions
Does the ebullioscopic constant depend on the solute?
No. Kb is a property of the solvent alone; the identity of the solute does not affect its value, only how many particles it contributes per mole (the van't Hoff factor).
How is Kb used to find molar mass?
By measuring ΔTb for a known mass of solute dissolved in a known mass of solvent, the molality can be calculated from m = ΔTb/Kb, and then the solute's molar mass is found from moles = mass of solute / (m × kg of solvent).