Freezing Point Depression Calculator
Calculate how much a solute lowers the freezing point of a solvent using ΔTf = i·Kf·m.
How it works
ΔTf = i · Kf · m · new freezing point = 0 °C − ΔTf (water)
A dissolved solute lowers a liquid's freezing point — the principle behind salting icy roads and antifreeze. Like boiling-point elevation it is colligative, depending on the number of particles. Kf is the cryoscopic constant (1.86 °C·kg/mol for water) and the van 't Hoff factor i counts particles per formula unit.
Worked example
A 1 mol/kg solution of a non-electrolyte (i = 1) with Kf = 1.86: ΔTf = 1 × 1.86 × 1 = 1.86 °C, so water freezes at about −1.86 °C.
Frequently asked questions
Why does salt melt ice on roads?
Salt dissolves in the thin water layer and lowers its freezing point, so ice melts at temperatures below 0 °C. Calcium chloride works to even lower temperatures because it releases three ions per unit.
How do I choose the van 't Hoff factor?
Use 1 for molecular solutes like sugar or ethanol, 2 for salts that give two ions such as NaCl, and 3 for those giving three ions such as CaCl₂ or Na₂SO₄.
What is Kf for other solvents?
It varies: 1.86 °C·kg/mol for water, 5.12 for benzene and 40 for camphor. Camphor's huge Kf once made it useful for measuring molar masses by freezing point.
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