Colligative Properties Solutions Colligative Properties

Colligative Properties Properties that depend on the concentration of solute particles but NOT on their identity Nonvolatile substance: A compound that has little tendency to become a gas under existing conditions

Vapor – Pressure Lowering The pressure of a vapor in equilibrium with its liquid phase Can be thought of as a measure of the particles tendency to escape into the vapor phase How does the vapor pressure of a solvent change when it contains a nonvolatile solute? It is lowered

Freezing – Point Depression Molal freezing-point constant (Kf): The freezing-point depression in a 1-molal solution of a nonvolatile, nonelectrolyte solute Freezing-point depression (∆tf): The difference between the freezing points of the pure solvent and a solution of nonelectrolyte in that solvent

Freezing – Point Depression molality Measured in oC/molal Measured in oC

Freezing – Point Depression Example: A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of -0.23 oC. What is the molal concentration of the solution? Kf = -1.86 oC/m for water

Freezing – Point Depression Example: A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of -0.23 oC. What is the molal concentration of the solution?

Boiling – Point Elevation Molal boiling-point constant (Kb): The boiling-point elevation in a 1-molal solution of a nonvolatile, nonelectrolyte solute Boiling-point elevation (∆tb): The difference between the boiling points of the pure solvent and a solution of nonelectrolyte in that solvent

Boiling – Point Elevation molality Measured in oC/molal Measured in oC

Boiling – Point Elevation Example: What is the boiling-point elevation of a solution made from 20.0 g of a nonelectrolyte solute and 400.0 g of water? The molar mass of the solute is 62.0 g/mol. Kf = 0.51 oC/m for water

Boiling – Point Elevation Example: What is the boiling-point elevation of a solution made from 20.0 g of a nonelectrolyte solute and 400.0 g of water? The molar mass of the solute is 62.0 g/mol. Step 1: find the molality

Boiling – Point Elevation Example: A water solution containing an unknown quantity of a nonelectrolyte solute is found to have a freezing point of -0.23 oC. What is the molal concentration of the solution?