Freezing Point Depression Calculator
Determine the freezing point depression caused by dissolved solutes using the linear colligative property equation.
Water freezes at 0 C under 1 atm pressure.
Water has Kf near 1.86 C kg per mol.
Strong electrolytes have i greater than 1 due to dissociation.
Freezing point depression
3.72 C
Solution freezing point
-3.72 C
How to Use This Calculator
Record solvent data
Enter the pure solvent freezing point and its cryoscopic constant.
Determine solute molality
Compute moles of solute per kilogram of solvent for the solution of interest.
Estimate van't Hoff factor
Use the number of particles formed per solute unit. Non-electrolytes typically have i equal to 1.
Review the outputs
DeltaTf equals i times Kf times m. Subtract DeltaTf from the pure solvent freezing point to obtain the solution freezing point.
Formula
DeltaTf = i * Kf * m
Solution freezing point Tf(solution) = Tf(solvent) - DeltaTf. Kf units: C kg per mol. Molality units: mol per kg.
Example
For a 2 molal NaCl solution (i about 2): DeltaTf = 2 * 1.86 * 2 = 7.44 C, so the solution freezes near -7.44 C.
Full Description
Freezing point depression occurs because dissolved solutes lower the chemical potential of the liquid phase, requiring colder temperatures to crystallize.
The magnitude depends on the number of dissolved particles and the solvent response captured by Kf. This linear relation holds for dilute solutions.
Frequently Asked Questions
Is molality the same as molarity?
No. Molality divides by solvent mass, while molarity divides by solution volume.
Why is i less than the ideal value?
Real electrolytes may incompletely dissociate or form ion pairs, reducing the effective van't Hoff factor.
Can this handle antifreeze mixtures?
Yes, provided the solution is dilute and you know the effective molality and i value.
How does pressure affect results?
Pressure effects are small at ambient conditions and typically ignored in the cryoscopic equation.
Where can I find Kf values?
Reference handbooks list cryoscopic constants for many solvents and temperatures.