Freezing Point Depression Calculator
ChemistryCalculate freezing point depression for solutions using ΔTf = Kf × m × i. Find new freezing point for water, benzene, and other solvents with any solute.
1.86
Freezing Point Depression ΔTf (°C)
1.86
New Freezing Point (°C)
-1.86
New Freezing Point (K)
271.29
What is a FP Depression?
The Freezing Point Depression Calculator computes the decrease in freezing point when a solute is dissolved in a solvent, using ΔTf = Kf × m × i. Select the solvent (with preset cryoscopic constant Kf), enter the molality and the van't Hoff factor, and get the freezing point depression and the new freezing point of the solution.
Freezing point depression is one of the four colligative properties of solutions, alongside boiling point elevation (computed by the Boiling Point Elevation Calculator), osmotic pressure, and vapour pressure lowering. All four arise from the same root cause: dissolved solute particles reduce the chemical potential of the solvent in the liquid phase, destabilising the solid relative to the liquid. For freezing point depression specifically, this means the liquid-solid equilibrium temperature is lowered — the solution must be cooled more than the pure solvent to solidify.
The magnitude depends only on the number of particles per kilogram of solvent (m × i), not on what those particles are. This is why the formula has the same structure whether the solute is salt, sugar, or antifreeze — and why measuring ΔTf allows back-calculation of the solute's molar mass.
How to use this FP Depression calculator
- Select the Solvent from the dropdown. The Kf value and normal freezing point are preset for Water (Kf = 1.86), Benzene (5.12), Camphor (37.7), Cyclohexane (20.2), and Acetic Acid (3.9). For other solvents, select Custom Kf and enter the value.
- Enter the Molality in mol/kg — moles of solute per kilogram of pure solvent. For molar mass problems: molality = (mass of solute / molar mass) / (mass of solvent in kg).
- Enter the van't Hoff Factor (i): 1 for sugars and alcohols, 2 for NaCl/KCl, 3 for CaCl₂/Na₂SO₄, 4 for FeCl₃, etc.
- Read ΔTf (°C) and the New Freezing Point.
- For molar mass determination: rearrange to find m = ΔTf / (Kf × i), then molar mass = (mass of solute) / (m × mass of solvent in kg).
Formula & Methodology
Freezing point depression:ΔTf = Kf × m × i T_f(solution) = T_f(pure solvent) − ΔTfMolar mass determination from ΔTf:m = ΔTf / (Kf × i) M₂ = w₂ / (m × w₁) [w₂ = mass of solute in g, w₁ = mass of solvent in kg]Common Kf values: | Solvent | Freezing Point (°C) | Kf (°C·kg/mol) | |---|---|---| | Water | 0.0 | 1.86 | | Benzene | 5.5 | 5.12 | | Camphor | 179.8 | 37.7 | | Cyclohexane | 6.5 | 20.2 | | Acetic Acid | 16.6 | 3.9 | Worked example — molar mass by cryoscopy: 2.5 g of an unknown non-electrolyte (i = 1) dissolved in 50 g (0.05 kg) of benzene. Measured ΔTf = 0.640°C. Kf(benzene) = 5.12.m = ΔTf / (Kf × i) = 0.640 / (5.12 × 1) = 0.125 mol/kg Moles of solute = m × kg solvent = 0.125 × 0.05 = 0.00625 mol M₂ = 2.5 g / 0.00625 mol = 400 g/molThe unknown compound has a molar mass of 400 g/mol — consistent with a small polymer, natural product, or organic compound of moderate size.
Frequently Asked Questions
Freezing point depression is a colligative property — the phenomenon where dissolving a solute in a solvent lowers the solution's freezing point below that of the pure solvent. The depression occurs because dissolved particles disrupt the formation of the ordered crystal lattice of the solid phase, making it harder for the liquid to solidify at the normal freezing temperature. Like all colligative properties, the effect depends on the number of dissolved particles, not their chemical identity.
The freezing point depression is ΔTf = Kf × m × i, where Kf is the cryoscopic constant of the solvent (°C·kg/mol), m is the molality (mol of solute per kg of solvent), and i is the van't Hoff factor. For water, Kf = 1.86 °C·kg/mol. The new freezing point is T_f(solution) = T_f(pure solvent) − ΔTf. The negative sign is important: freezing point depression means the freezing point is lowered, so we subtract ΔTf from the pure solvent freezing point.
Sodium chloride (NaCl) dissolves in liquid water on ice surfaces and creates a solution with a depressed freezing point. If the temperature is above the new freezing point of the salt-water solution, the ice melts rather than refreezing. NaCl is effective down to about −9°C (using Kf = 1.86, molality ≈ 5 mol/kg, i = 2: ΔTf = 18.6°C — so effective down to −18.6°C at saturation). Below −9°C, MgCl₂ or CaCl₂ (Kf × m × i larger due to i=3) are used. In Himachal Pradesh and J&K, road salt is applied to Himalayan highways.
The cryoscopic constant Kf (also called the molal freezing point depression constant or cryoscopic constant) is a solvent-specific property that quantifies how much 1 mol/kg of a non-electrolyte solute lowers the freezing point. Key values: Water: 1.86 °C·kg/mol; Benzene: 5.12 °C·kg/mol; Camphor: 37.7 °C·kg/mol (extremely high — used for Rast method molar mass determination); Cyclohexane: 20.2 °C·kg/mol. Higher Kf means a larger depression per mol/kg, making the solvent better for molar mass determination via cryoscopy.
The cryoscopic method (Beckmann method) determines molar mass by measuring ΔTf: dissolve a known mass (w₂) of the unknown solute in a known mass (w₁) of solvent and measure ΔTf precisely. Then m = ΔTf/Kf and M₂ = (w₂ × 1000) / (m × w₁). Using camphor (Kf = 37.7) gives large, easily measured ΔTf values for small amounts of solute. For example, dissolving 1 g of an unknown compound in 20 g camphor and measuring ΔTf = 1.885°C gives M₂ = (1 × 1000) / (0.05 × 20) = 1000 g/mol.
Select the solvent (Water, Benzene, Camphor, Cyclohexane, Acetic Acid, or Custom Kf). Enter the molality in mol/kg — moles of solute per kilogram of solvent. Enter the van't Hoff factor i: 1 for non-electrolytes, 2 for NaCl, 3 for CaCl₂, etc. The calculator returns ΔTf and the new freezing point in °C and K.
Both are colligative properties caused by the same underlying mechanism — lowered vapour pressure of the solution due to solute particles. Freezing point depression lowers the freezing point: ΔTf = Kf × m × i. Boiling point elevation raises the boiling point: ΔTb = Kb × m × i. The Kf for water (1.86) is much larger than Kb for water (0.512), so the same solution shows a larger freezing point depression than boiling point elevation. Use the [Boiling Point Elevation Calculator](/boiling-point-elevation-calculator/) for the complementary calculation.
Seawater with salinity of 35 g/kg contains predominantly NaCl (≈27.2 g/kg) and other salts. The effective ion molality is approximately 1.1 mol/kg. Freezing point depression: ΔTf = 1.86 × 1.1 = 2.05°C. Seawater freezes at approximately −2°C rather than 0°C. This is why the North Atlantic and polar oceans remain liquid at temperatures slightly below 0°C. Sea ice that does form has a lower salt content than the surrounding water because salt is expelled from the growing crystal lattice during freezing.
Ethylene glycol antifreeze is a non-electrolyte (i = 1) but is used in high concentrations. A 50% by mass ethylene glycol-water mixture has a molality of approximately 16.1 mol/kg (molecular weight of ethylene glycol = 62 g/mol). Freezing point depression: ΔTf = 1.86 × 16.1 × 1 = 30°C. This lowers the freezing point to −30°C. The 50/50 mix is the standard recommendation for most climates, providing protection to −37°C including van't Hoff corrections for activity coefficients at high concentration.
Yes — freezing point depression is important in Indian ice cream manufacturing, dairy frozen desserts, and cold storage. Ice cream mix formulations balance sucrose, lactose, and salts to achieve a target freezing point that gives the right scoopability texture. Indian standards (FSSAI) specify ice cream composition, and formulation of kulfis, ice creams, and sorbets requires calculating the effective freezing point to achieve the desired hardness and melting rate. In food distribution and cold chain logistics, knowing the freezing point of brines used for indirect cooling is critical for HACCP compliance.