Chapter 21 Colligative Properties: Boiling Point & Freezing Point

Boiling Point Elevation Boiling point elevation is a colligative property related to vapor pressure lowering. The boiling point is defined as the temperature at which the vapor pressure of a liquid equals the atmospheric pressure. Due to the lowering of the vapor pressure of the solvent when a solution forms, a solution will require a higher temperature than the pure solvent to reach its boiling point.

Boiling Point Elevation ∆T b = (K b )(m)(#particles) # particles Molecular solutes always = 1 Example: C 6 H 12 O 6 Ionic solutes = # of ions in the compound Example: BaCl 2, Al 2 (SO 4 ) 3

What is the boiling point of a 2.4m aqueous solution Ba(NO 3 ) 2 ? BP = °C

Freezing Point Depression Every liquid has a freezing point - the temperature at which a liquid undergoes a phase change from liquid to solid. When solutes are added to a liquid, forming a solution, the solute molecules disrupt the formation of crystals of the solvent. That disruption in the freezing process results in a depression of the freezing point for the solution.

Freezing Point Depression ∆T f = (K f )(m)(#particles)

What is the freezing point of a 1.5m aqueous solution of NaCl? FP = °C

97.5g of C 12 H 22 O 11 are dissolved in 185g of water. Find the freezing point and boiling point. FP = °C BP = °C

14.0g of C 10 H 8 are dissolved in 25.0g of benzene. Find the freezing point and boiling point. FP = °C BP = °C

Homework Finish Worksheet: Vapor Pressure, Boiling Point and Freezing Point. Review and Reinforcement worksheet due tomorrow.