❄️Freezing Point Depression (ΔTf=iKfm)
Water=1.86
NaCl=2, glucose=1
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.
- 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)
1 mol NaCl in 1 kg water · i=2 · Kf=1.86=ΔTf = 3.72°C → freezes at −3.72°C2×1.86×1=3.72°C depression
⭐
Fun Fact
Road salt (NaCl) works by freezing point depression, but only down to about −9°C. At lower temperatures, CaCl₂ (which gives i=3 and larger ΔTf) is used for de-icing.
References
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