# How is freezing point depression related to molality?

Jan 3, 2018

Boiling point elevation and freezing point depression are colligative properties. Colligative properties are physical properties of solutions that change depending on the concentration of solute we add to the solvent and that means the volume of the solution is also changing.

Thus, the temperature of the solution changes according to the molality.

You can calculate the freezing point point depression $\Delta {T}_{f}$ from the following formula:

$\Delta {T}_{f} = - i {K}_{f} m$, where,

• $\Delta {T}_{f} = {T}_{f} - {T}_{f}^{\text{*}}$ is the final freezing point (solution) minus the initial freezing point (solvent), and is always negative. ($\text{*}$ indicates pure solvent.)
• $m$ is the molal concentration of the solution. ( molality = moles of solute / kg of solvent.)
• ${K}_{f}$ is the freezing point depression constant and has a positive sign.
• $i$ is the van't Hoff factor, or the effective number of particles each solute particle dissociates to give, which can account for ion pairing in more precise work.