Freezing Point Depression Calculator

Calculate freezing point depression using delta Tf = Kf * m * i. Find new freezing point of solutions for antifreeze, road salt, and cryoscopy applications.

🧊 Colligative📐 ΔTf = Kf·m·i🧪 Chemistry

Molality m (mol solute/kg solvent)

Kf of solvent (°C·kg/mol)

van't Hoff factor i

Normal freezing point T° (°C)

Please enter valid values.

Formula & Reference

Variable	Symbol	Formula	Units
Freezing Point Depression Calculator	—	ΔTf = Kf·m·i	°C

Step-by-Step Examples

Example 1

NaCl on Roads

1.00 mol/kg NaCl (i=2) in water (Kf=1.86).

dTf = 1.86 x 1.00 x 2 = 3.72 C
New freezing point: 0.00 - 3.72 = -3.72 C

✓ Ice melts at -3.72 C with this salt concentration

Example 2

Antifreeze

4.00 mol/kg ethylene glycol (i=1, Kf=1.86) in water.

dTf = 1.86 x 4.00 x 1 = 7.44 C
Freezing point: 0.00 - 7.44 = -7.44 C
50/50 mix gives ~-37 C protection

✓ Freezing point -7.44 C per molal unit

Example 3

Molar Mass by Cryoscopy

0.500 g unknown in 10.0 g benzene (Kf=5.12). Freezing point drops 0.640 C.

m = dTf/(Kf x i) = 0.640/(5.12 x 1) = 0.125 mol/kg
n_solute = 0.125 x 0.0100 = 0.00125 mol
M = 0.500/0.00125 = 400 g/mol

✓ Molar mass = 400 g/mol

Real-World Applications

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Road De-Icing

NaCl (Kf=1.86, i=2) and CaCl2 (i=3) lower ice melting point. CaCl2 is more effective per gram.

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Antifreeze

Ethylene glycol in water protects engines from -37 C (50:50 mix) to -65 C (70:30 mix).

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Molar Mass Determination

Cryoscopy: measure freezing point depression to find molar mass of unknown substances.

🦻

Cryopreservation

Cryoprotective agents (DMSO, glycerol) depress freezing point to protect biological samples.

Common Mistakes to Avoid

⚠️

Depression is subtracted from T0

New Tf = T0 - dTf. Freezing point goes DOWN. New boiling point goes UP. Don't get directions confused.

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i factor for ionic compounds

NaCl: i=2, CaCl2: i=3, AlCl3: i=4. Non-electrolytes (glucose, ethanol): i=1. Real i slightly less than ideal.

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Kf values for solvents

Water: 1.86. Benzene: 5.12. Cyclohexane: 20.2. Camphor: 39.7 C-kg/mol. Camphor used for precise cryoscopy.

Frequently Asked Questions

Why does dissolving a solute lower the freezing point? ▾

Solute disrupts crystal lattice formation. Solvent molecules have less tendency to form solid because they are diluted by solute. Lower T needed to freeze.

Why is CaCl2 better than NaCl for de-icing? ▾

CaCl2 gives i=3 vs NaCl i=2. Same moles of salt gives 50% more freezing point depression. Also, CaCl2 dissolving is exothermic (generates heat).

What is cryoscopy? ▾

Determination of molar mass by measuring freezing point depression. M = (Kf x mass_solute)/(dTf x mass_solvent x i).

What is the eutectic point? ▾

Temperature at which a mixture has its lowest freezing point. NaCl-water eutectic: -21.2 C at 23.3% NaCl. Below this, salt cannot melt ice.

Can you freeze water below -21 C with salt alone? ▾

No. The NaCl-water eutectic is -21.2 C. Below this temperature, even a saturated NaCl solution freezes. Other salts (CaCl2) work to lower temperatures.

Related Chemistry Calculators

Formula Explorer connections

Interpretation: This formula connects solute amount and solution volume, mass or particle count to concentration and colligative behavior. Assumption: Distinguish solution volume from solvent volume, use the stated temperature, and account for dissociation or nonideal activity when required.

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