Freezing Point Depression Calculator
What This Calculator Does
The Freezing Point Depression Calculator calculates how much a solvent's freezing point decreases when a solute is dissolved in it. This is a colligative property, meaning it depends on the number of dissolved particles, not their chemical identity.
When you add antifreeze to water, salt to ice, or any solute to a solvent, the freezing point drops. This calculator uses the formula ΔTf = Kf × m × i to calculate this depression, where:
- ΔTf - Freezing point depression (temperature decrease in °C or K)
- Kf - Cryoscopic constant (specific to each solvent)
- m - Molality of the solution (mol solute/kg solvent)
- i - Van't Hoff factor (number of particles per formula unit)
Formula & Calculation Method
Core Formula
Where:
- ΔTf = Freezing point depression (°C)
- Kf = Cryoscopic constant (°C·kg/mol)
- m = Molality (mol/kg)
- i = Van't Hoff factor (dimensionless)
Common Kf Values:
Water: 1.86 °C·kg/mol
Benzene: 5.12 °C·kg/mol
Cyclohexane: 20.0 °C·kg/mol
Acetic acid: 3.90 °C·kg/mol
Van't Hoff Factor (i):
Non-electrolytes (sugar, ethylene glycol): i = 1
NaCl: i ≈ 2 (Na⁺ + Cl⁻)
MgCl₂: i ≈ 3 (Mg²⁺ + 2Cl⁻)
CaCl₂: i ≈ 3 (Ca²⁺ + 2Cl⁻)
Step-by-Step Example
Problem: What is the freezing point of a solution made by dissolving 62 g of ethylene glycol (C₂H₆O₂) in 250 g of water?
Step 1: Calculate molality
Molar mass of C₂H₆O₂ = 62 g/mol
Moles = 62 g ÷ 62 g/mol = 1.00 mol
m = 1.00 mol ÷ 0.250 kg = 4.00 m
Step 2: Identify constants
Kf (water) = 1.86 °C·kg/mol
i (ethylene glycol) = 1 (non-electrolyte)
Step 3: Calculate ΔTf
ΔTf = 1.86 × 4.00 × 1 = 7.44 °C
Step 4: Find new freezing point
Normal FP of water = 0.0 °C
New FP = 0.0 - 7.44 = -7.44 °C
Real-World Applications
🚗 Antifreeze
Ethylene glycol in car radiators prevents coolant from freezing in winter. A 50/50 mixture provides protection down to -37°C (-34°F).
🧂 De-icing
Salt (NaCl, CaCl₂) on roads melts ice by lowering freezing point. CaCl₂ is more effective because it produces 3 ions and works at lower temperatures.
🍦 Ice Cream
Salt added to ice in hand-cranked ice cream makers lowers temperature below 0°C, allowing cream mixture to freeze properly.
🔬 Research
Determine molecular mass of unknown compounds. Cryoscopy is used in physical chemistry laboratories for molecular mass determination.
Quick Reference
Units
°C, mol/kg
Formula
ΔTf = Kf × m × i
Applications
Antifreeze, de-icing
Level
College chemistry
Related Calculators
Boiling Point Elevation
Calculate ΔTb
Molality Calculator
Calculate molal concentration
Solution Preparation
Prepare solutions
Related Formulas
Where It's Used
Automotive
Engine antifreeze
Winter Roads
De-icing salt
Food Industry
Ice cream making
Research
Molecular mass