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Freezing point depression is a colligative property: adding a solute to a pure solvent lowers the freezing point. ΔTf = i × Kf × m, where i is the van't Hoff factor, Kf is the cryoscopic constant, and m is molality.
Trinn-for-trinn guide
- 1ΔTf = i × Kf × m (molality, not molarity)
- 2Kf(water) = 1.86 °C·kg/mol
- 3Molality m = moles solute / kg solvent
- 4van't Hoff factor i = number of particles per formula unit (1 for glucose, 2 for NaCl, 3 for CaCl₂)
Løste eksempler
Inndata
1 mol NaCl in 1 kg water · i=2 · Kf=1.86
Resultat
ΔTf = 3.72°C → freezes at −3.72°C
2×1.86×1=3.72°C depression
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