What is freezing point depression?

It is the amount by which a dissolved solute lowers the freezing point of a solvent. It is a colligative property, meaning it depends on the number of dissolved particles rather than what they are, which is why salt and sugar both depress freezing but to different degrees per gram.

What is the van't Hoff factor?

The van't Hoff factor i is the number of particles a solute produces in solution. Non-electrolytes such as glucose give i around 1, sodium chloride gives about 2, and calcium chloride about 3. At higher concentrations the effective i drops slightly because of ion pairing.

How is osmolality related?

Osmolality is the total particle concentration, equal to i times the molality, expressed in osmoles per kilogram. Because freezing point depression is proportional to particle count, osmometers measure it precisely to report osmolality, which matters for IV fluids and tears.

Why does my measured value differ from the calculation?

Real ionic solutions deviate from the ideal van't Hoff factor at higher concentrations due to ion pairing and non-ideal behaviour, so the measured depression is usually a little smaller than calculated. The formula is most accurate in dilute solutions.

Freezing Point Depression Calculator

Q: Which formula does the tool use?

It uses delta Tf equals i times Kf times molality, where Kf is the solvent's cryoscopic constant, molality is moles of solute per kilogram of solvent, and i is the van't Hoff factor counting particles released per formula unit. The new freezing point is the normal point minus delta Tf.

When you dissolve something in a liquid, it freezes at a lower temperature than the pure solvent. This freezing point depression is one of the four colligative properties, and it underpins everything from de-icing roads to formulating isotonic eye drops and protecting cells during cryopreservation.

How it works

The depression is proportional to the concentration of dissolved particles:

delta Tf = i x Kf x m
new fp   = normal fp - delta Tf
osmolality = i x m

Here Kf is the cryoscopic constant of the solvent, for water 1.86 degrees Celsius per molal, m is the molality in moles of solute per kilogram of solvent, and i is the van’t Hoff factor counting how many particles each formula unit releases. The osmolality, the total particle concentration, follows directly as i times molality.

Notes and example

A one molal solution of sodium chloride in water, with i near 2, depresses the freezing point by about 3.7 degrees, freezing near minus 3.7 Celsius, and has an osmolality of roughly 2 osmoles per kilogram. The formula is most accurate in dilute solutions; concentrated ionic solutions show a slightly smaller depression than predicted because ion pairing reduces the effective particle count. Choose the van’t Hoff factor to match the solute, and remember the property depends on particle number, not chemical identity.