Chemistry · General chemistry II · Worked example
Freezing and boiling points of an ethylene glycol solution
25.0 g of ethylene glycol, C₂H₆O₂, is dissolved in 100.0 g of water. Find the freezing and boiling points of the solution. For water, Kf = 1.86 °C·kg/mol and Kb = 0.512 °C·kg/mol.
Moles of solute
Ethylene glycol’s molar mass is 2(12.01) + 6(1.008) + 2(16.00) = 62.07 g/mol. It is a nonelectrolyte, so i = 1.
Molality
Divide by kilograms of solvent, not by the solution’s mass or volume.
Freezing-point depression
The freezing point drops below 0.00 °C by ΔTf.
Boiling-point elevation
The boiling point rises above 100.00 °C by ΔTb.
Result
The solution freezes at −7.49 °C and boils at 102.06 °C (at 1 atm, treating it as ideal).
Your turn
What is the freezing point of 0.100 m CaCl₂(aq), assuming ideal dissociation?
Show the answer and explanation
About −0.558 °C.
CaCl₂ gives three ions, so i = 3. ΔTf = 3 × 1.86 × 0.100 = 0.558 °C, so the solution freezes at −0.558 °C. The measured value is a little smaller in magnitude because of ion pairing.
Keep exploring
Open the Colligative studio with this solution, switch to boiling-point elevation, then try i = 2 to see what an ionic solute would do.
Return to the concept →Sources and scope
Authored study material. Tool results depend on the stated inputs and model assumptions.
- Tro, Chemistry: A Molecular Approach, 4th ed., §13.5 Expressing Solution Concentration, pp. 585–592 (molality, p. 588)
- Tro, Chemistry: A Molecular Approach, 4th ed., §13.6 Colligative Properties: Vapor Pressure Lowering, Freezing Point Depression, Boiling Point Elevation, and Osmotic Pressure, pp. 593–605 (volatile solutes, p. 597; osmotic pressure, pp. 603–604)
- Tro, Chemistry: A Molecular Approach, 4th ed., §13.7 Colligative Properties of Strong Electrolyte Solutions, pp. 605–607
- OpenStax Chemistry 2e — Colligative properties