Freezing Point Depression Calculator

Freezing Point Depression Calculator

The Freezing Point Depression Calculator applies colligative property theory to find how a solute lowers the freezing point of a solvent. Adding a nonvolatile solute reduces vapor pressure and lowers the freezing point below that of the pure solvent.

$$\Delta T_f = i \cdot K_f \cdot m$$

where $\Delta T_f$ is the freezing point depression in °C, $i$ is the van't Hoff factor, $K_f$ is the molal freezing point depression constant (°C·kg/mol), and $m$ is molality in mol/kg.

The solution freezing point is:

$$T_f^{\text{solution}} = T_f^{\text{pure}} - \Delta T_f$$

Example: a $0.4$ mol/kg ethylene glycol solution in water ($K_f = 1.86$ °C·kg/mol, $i = 1$) gives $\Delta T_f = 0.744$ °C and $T_f^{\text{solution}} = -0.74$ °C.

Road salt works the same way: dissolved ions ($i > 1$) depress the freezing point of water on roads and sidewalks.

Related tools: Concentration Calculator and Chemical Entropy Calculator.