# Freezing Point Depression Calculator

Calculate freezing point depression with ΔTf = iKfm. Supports water, benzene, cyclohexane, and custom solvents. Free colligative property calculator.

## What this calculates

Calculate how much the freezing point of a solvent decreases when a solute is dissolved in it. Uses the colligative property formula ΔTf = i × Kf × m, where i is the van't Hoff factor, Kf is the cryoscopic constant, and m is molality.

## Inputs

- **Van't Hoff Factor (i)** — min 1, max 10 — Number of particles the solute dissociates into (1 for non-electrolytes, 2 for NaCl, 3 for CaCl₂).
- **Molality (m)** (mol/kg) — min 0 — Moles of solute per kilogram of solvent.
- **Solvent** — options: Water (Kf = 1.86 °C/m, FP = 0 °C), Benzene (Kf = 5.12 °C/m, FP = 5.5 °C), Cyclohexane (Kf = 20.0 °C/m, FP = 6.5 °C), Custom solvent — Select a solvent or choose custom to enter your own Kf value.
- **Custom Kf** (°C/m) — min 0 — Cryoscopic constant of the solvent. Only used when 'Custom solvent' is selected.
- **Custom Normal Freezing Point** (°C) — Normal freezing point of the pure solvent in °C. Only used when 'Custom solvent' is selected.

## Outputs

- **ΔTf (Depression)** (°C) — The freezing point depression in degrees Celsius.
- **New Freezing Point** (°C) — The new freezing point of the solution.
- **Explanation** — formatted as text — Step-by-step calculation breakdown.

## Details

Freezing point depression is a colligative property describing how the addition of solute to a solvent lowers the temperature at which the solution freezes. Like boiling point elevation, it depends only on the number of dissolved particles, not the nature of the solute.

The Formula: ΔTf = i × Kf × m

- i = van't Hoff factor (number of particles per formula unit upon dissolution)

- Kf = cryoscopic constant (unique to each solvent). Water: 1.86 °C·kg/mol, Benzene: 5.12 °C·kg/mol

- m = molality (moles of solute per kg of solvent)

- The new freezing point = normal freezing point − ΔTf

Practical Applications

Road salt (NaCl or CaCl₂) lowers the freezing point of water on roads, preventing ice formation. Automotive antifreeze (ethylene glycol) depresses the freezing point of engine coolant to well below 0 °C. The cryoscopic method is a classical technique for determining the molecular weight of an unknown solute by measuring how much it depresses the freezing point.

Comparison of Solvents

Cyclohexane has a very high Kf (20.0 °C·kg/mol), making it especially sensitive for molecular weight determination. Benzene (Kf = 5.12) is moderately sensitive. Water (Kf = 1.86) shows smaller depressions but is the most practically important solvent.

## Frequently Asked Questions

**Q: Why does salt lower the freezing point of water?**

A: When salt dissolves in water, it creates ions (Na⁺ and Cl⁻) that interfere with the formation of the crystalline ice lattice. Water molecules must be at a lower temperature to overcome the disruption caused by these dissolved particles and form ice.

**Q: How much antifreeze do I need to protect my car?**

A: A 50/50 mix of ethylene glycol (MW = 62.07, i = 1) and water is standard automotive coolant. At about 8.06 molal, this gives ΔTf = 1 × 1.86 × 8.06 ≈ 15 °C depression, plus non-ideal solution effects bring the actual freezing point to about -37 °C.

**Q: Why is cyclohexane preferred for molecular weight determination?**

A: Cyclohexane has the highest Kf among common solvents (20.0 °C·kg/mol), meaning a small amount of solute produces a large, easily measurable freezing point depression. This makes the cryoscopic method more precise for determining unknown molecular weights.

**Q: Is CaCl₂ a better de-icer than NaCl?**

A: CaCl₂ has a van't Hoff factor of 3 (Ca²⁺ + 2Cl⁻) compared to 2 for NaCl. Per mole, CaCl₂ produces 50% more particles and thus a greater freezing point depression. CaCl₂ is effective down to about -29 °C, while NaCl only works to about -21 °C.

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Source: https://vastcalc.com/calculators/chemistry/freezing-point-depression
Category: Chemistry
Last updated: 2026-04-21