Freezing Point Depression and Boiling Point Elevation

Freezing Point Depression

• Definition: The freezing point of a solution is lower than the freezing point of the pure solvent.

  • Melting point of the solution is also lower.

• Formula:
  [ \Delta TF = T{F{solvent}} - T{F{solution}} = m \cdot KF ]

  • ( \Delta T_F ): change in freezing point
  • ( m ): molality of solute particles
  • ( K_F ): freezing point depression constant for the solvent

• Example Calculation:

  • Given:
  • Molality (m) = 1.7 m
  • Freezing point depression constant (K_F) = 1.86 °C/m
  • Calculation:
  • ( \Delta T_F = 1.7 \times 1.86 = 3.2 °C )
  • Freezing point of the solution:
    • ( 0° C - 3.2° C = -3.2° C )

Boiling Point Elevation

• Definition: The boiling point of a solution is higher than that of the pure solvent when a non-volatile solute is dissolved in it.

• Formula:
  [ \Delta TB = T{B{solution}} - T{B{solvent}} = m \cdot KB ]

  • ( \Delta T_B ): change in boiling point
  • ( m ): molality of solute particles
  • ( K_B ): boiling point elevation constant for the solvent

• Example Calculation:

  • Goal: Determine the mass of ethylene glycol (C₂H₂O₂) required to produce a solution that boils at 105° C
  • Given:
  • Mass of solvent (water) = 1 kg
  • Boiling point elevation constant (K_B) for water (assumed to be in the context)
  • Adjustments would be needed based on K_B value and other details.