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Chapter 14: Ions in Aqueous Solutions and Colligative Properties Section 2: Colligative Properties of Solutions Overview • We will examine what colligative properties are. • We will define the main colligative properties. What Are They? • Colligative Properties – Properties that depend on the concentration of solute particles, but not on their identity. • Non-volatile Substance – One that has little tendency to become a gas under existing conditions. Vapor-Pressure Lowering • By adding certain solutes the vapor pressure of some solvents can be lowered. Freezing Point Depression • The solution also lowers the freezing point of the solvent alone. • Molal Freezing Point Constant – (Kf) It is for one molal (m) of a nonvolitile, nonelectrolyte solute. • To calculate the freezing point depression we use the following equation: • Δtf = Kf m • Kf = oC/m • m = molality Boiling Point Elevation • By adding solute it causes the boiling point of the solvent to rise. • Molal Boiling Point Constant – (Kb) It is for one molal (m) of a nonvolitile, nonelectrolyte solute. • Δtb = Kb m • Kb = oC/m • m = molality Osmotic Pressure • Semipermeable Membrane – A membrane that allows the movement of some particles while blocking the movement of others. • Osmosis – The movement of solvent through a semipermeable membrane from the side of lower solute concentration to the side of higher solute concentration. • Osmotic Pressure – The external pressure that must be applied to stop osmosis. Electrolytes • Electrolytes tend to depress the freezing point greatly. • They also tend to elevate the boiling point greatly as well. • That is why we use salt to get rid of ice or to prevent the formation of ice.
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