Freezing point depression constant

Definition

The freezing point depression constant ($K_f$) is a proportionality factor that relates the decrease in the freezing point of a solvent to the molality of the solute. It is specific to each solvent and is used in calculating colligative properties.

5 Must Know Facts For Your Next Test

1. $K_f$ has units of $^{\circ}C/m$, where $m$ represents molality.
2. Freezing point depression occurs because solute particles disrupt the formation of a solid phase.
3. $K_f$ values are experimentally determined for each solvent.
4. The formula for freezing point depression is $\Delta T_f = K_f \cdot m \cdot i$, where $i$ is the van't Hoff factor.
5. $K_f$ is typically found in tables or can be calculated using experimental data.

Review Questions

• What does the freezing point depression constant ($K_f$) signify?
• How do you calculate the change in freezing point using $K_f$?
• Why does adding a solute lower the freezing point of a solvent?

Related terms

Molality: A concentration unit defined as moles of solute per kilogram of solvent.

Van't Hoff Factor: A number that represents how many particles a compound dissociates into when dissolved.

Colligative Properties: Properties that depend on the number of solute particles but not their identity, including boiling point elevation, freezing point depression, vapor pressure lowering, and osmotic pressure.