❄️ Freezing Point Depression Calculator
Find how much a solute lowers the freezing point: ΔTf = i·Kf·m, or solve for molality, i, or the cryoscopic constant.
Freezing-point drop ΔTf
1.86 °C
Freezing-point depression: ΔTf = i·Kf·m. Water Kf = 1.86 °C·kg/mol. i = van’t Hoff factor, m = molality. 🔒 Computed in your browser.
How the freezing point depression calculator works
Freezing-point depression is a colligative property: ΔTf = i·Kf·m, where i is the van’t Hoff factor (particles per formula unit), Kf the cryoscopic constant (1.86 °C·kg/mol for water), and m the molality. Enter any three and the tool solves for the fourth. Rearranged, m = ΔTf ÷ (i·Kf), and the solution’s freezing point is the pure solvent’s minus ΔTf, for water, 0 °C − ΔTf.
Why salt melts ice and antifreeze works. For ionic solutes use i = number of ions (NaCl ≈ 2, CaCl₂ ≈ 3); for molecular solutes like sugar, i = 1.
Frequently asked questions
What is freezing point depression?
The lowering of a solvent’s freezing point when a solute is dissolved in it. It depends on the number of dissolved particles, not their identity, a colligative property. ΔTf = i·Kf·m.
What is the van’t Hoff factor (i)?
The number of particles a solute splits into. NaCl gives i ≈ 2 (Na⁺ + Cl⁻), CaCl₂ ≈ 3, and non-electrolytes like glucose give i = 1.
What is Kf for water?
The cryoscopic constant of water is 1.86 °C·kg/mol. Other solvents have their own Kf, which you can enter.
How do I find the new freezing point?
Subtract ΔTf from the pure solvent’s freezing point. For water, 0 °C − ΔTf. The tool gives ΔTf; subtract it yourself.
Why does salt melt ice?
Dissolved salt depresses the freezing point below the ambient temperature, so the ice melts. More particles (higher i·m) means a larger depression.
What freezing point does 2 m CaCl₂ give in water?
ΔTf = i·Kf·m = 3 × 1.86 × 2 = 11.16 °C, so the solution freezes near −11.2 °C. CaCl₂’s i ≈ 3 (one Ca²⁺ plus two Cl⁻) makes it a strong de-icer.
How is freezing-point depression used to find molar mass?
Measure ΔTf for a known mass of an unknown solute, back out the molality from ΔTf = Kf·m, convert to moles, then divide the solute mass by the moles. This “cryoscopy” yields the molar mass of the unknown.