16.3 Colligative Properties
16.3 Colligative Properties
Lesson Objectives
- Define a colligative property and identify three colligative properties of solutions.
- Differentiate between the effects that an electrolyte has on the colligative properties of a solution compared to
a nonelectrolyte. - Calculate the freezing and boiling points of a solution of known molality.
- Use freezing or boiling point information to calculate the molar mass of an unknown solute.
Lesson Vocabulary
- boiling point elevation
- colligative property
- freezing point depression
- molal boiling-point elevation constant
- molal freezing-point depression constant
Check Your Understanding
Recalling Prior Knowledge
- What is vapor pressure?
- How are melting and boiling points determined from a phase diagram?
Solvents have physical properties that can be affected by the process of dissolving a solute into the solvent. In this
lesson, you will learn how the vapor pressure, freezing point, and boiling point of a solvent change when a solution
is formed.
Vapor Pressure Lowering
Acolligative propertyis a property of a solution that depends only on the number of solute particles dissolved
in the solution and not on their identity. Recall that the vapor pressure of a liquid is determined by how easily its
molecules are able to escape the surface of the liquid and enter the gaseous phase. When a liquid evaporates easily,
it will have a relatively large number of its molecules in the gas phase and, thus, will have a high vapor pressure.
Liquids that do not evaporate easily have a lower vapor pressure. The image below (Figure16.10) shows the
surface of a pure solvent compared to a solution. In the picture on the left, the surface is entirely occupied by liquid
molecules, some of which will evaporate and form a vapor. On the right, a nonvolatile solute has been dissolved into