Freezing Point Depression Calculator
Estimate how a dissolved solute lowers the freezing point of a dilute solution.
01 Set up your calculation
- Enter moles of solute per kilogram of solvent.
- Supply the solvent constant and effective particle factor appropriate to the solution.
- Enter the pure solvent transition temperature at the pressure of interest.
02 The relationship
For the assumptions behind these results, read the explanation below. You can also explore Boiling Point Elevation and Molality.
Follow the numbers
For a 0.5 mol/kg nonelectrolyte solution in water, i = 1 and Kf = 1.86 K·kg/mol give a 0.93 K depression. Starting from 0 °C, the estimate is −0.93 °C.
03 Count dissolved particles on a solvent-mass basis
Molality means moles of solute per kilogram of solvent, not per liter of solution. This distinction matters because the temperature-shift equation is formulated on a solvent-mass basis. The solvent constant also belongs to the chosen solvent and transition.
The factor i adjusts the solute amount to an effective dissolved-particle amount. A nonelectrolyte often uses a starting approximation of one. An electrolyte’s effective value can differ from the ideal number of ions because interactions become important.
04 Treat the result as a dilute-solution estimate
The equation describes an initial freezing-point shift, not the full freezing behavior of a mixture. As solvent freezes out, the remaining liquid changes composition; the initial calculation no longer represents every later stage.
Water values are supplied as convenient starting inputs, not automatically selected constants for every solvent. The calculation becomes less dependable at high concentrations, near chemical changes, or when the assumed particle factor is inaccurate.
Questions from the notebook
Is a kelvin change the same size as a Celsius change?
Yes. A temperature interval of 1 K equals an interval of 1 °C, although absolute temperatures have different zero points.
Can I use molarity instead of molality?
Not directly. Convert using the appropriate solvent mass and solution composition first.
Does this account for pressure?
Only through the pure-solvent transition temperature you supply. It does not calculate that temperature from pressure.
References & reading
- OpenStax, Rice University — Colligative Properties ↗
Gives the molality relationships for freezing and boiling shifts and the dilute-solution osmotic-pressure equation.
- NIST — Guide to SI conversion factors ↗
Supports physical unit conversions. Input values are converted to the formula’s base units before calculation.
Our explanations are written for ChemistryCalc. Read how we build and check our tools. These calculations are educational estimates where the underlying model requires assumptions.