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Colligative Properties
The Physical Properties of Solutions
Colligative Properties
• There are 3 colligative or physical properties of solutions that you should know. They are Vapor pressure Boiling point elevation Freezing point depression Osmotic pressure is one you don’t need to worry about.
1. 2. 3. 4.
Molality
• Molality measures the moles of solute dissolved in the mass (kg) of solvent. • Molality normally expresses concentrations of colloids. • m = moles solute kg solvent
Molality
• Example: A 0.5 mole solute is prepared in 1 Kg of water. What is the molality of the solution? 0.5 moles = 0.5 molal 1 Kg of H2O
Molality
• Example: What is the molality of a solution prepared by dissolving 8.87 g of methanol CH3OH, in 20.0 g of water?
Calculate the moles of solute (CH3OH) 8.87 g = 0.277 mol 32.04 g
Molality
Calculate the molality m = 0.277 mol CH3OH x 1000 g solvent 20.0 g H2O 1 Kg solvent = 13.8 m
Molality
• Determine the molality of 3000 g of solution containing 37.3 g of KCl. 37.3 g KCl = 0.5 moles KCl 74.6 g
Molality
• Determine the grams of pure solvent. Total = mass of solute + mass of solvent 3000 g = 37.3 g + mass of solvent 3000 g – 37.3 g = 2962.7 g of solvent
Molality
• Convert grams of solvent to kilograms 2962.7 g x 1 Kg = 2.9627 Kg 1000 g • m = 0.5 moles = 0.169 m 2.9627 Kg
Changes in Vapor Pressure
• Nonvolatile solutes always lower the vapor pressure of a solution. • Vapor pressure (VP) is the result of evaporating solvent. • VP depends on temperature. • In a closed container, VP reaches a constant level at any given temperature.
Changes in Vapor Pressure
• Animation showing how VP is established. • Animation showing how VP is affected by temperature. • How does a solute affect VP?
Changes in Vapor Pressure
• Solvent molecules must be at the surface of the liquid to evaporate. • A surface molecule gets bumped from below and is “launched” into the vapor phase.
Solution Vapor
The Physical Properties of Solutions
Colligative Properties
• There are 3 colligative or physical properties of solutions that you should know. They are Vapor pressure Boiling point elevation Freezing point depression Osmotic pressure is one you don’t need to worry about.
1. 2. 3. 4.
Molality
• Molality measures the moles of solute dissolved in the mass (kg) of solvent. • Molality normally expresses concentrations of colloids. • m = moles solute kg solvent
Molality
• Example: A 0.5 mole solute is prepared in 1 Kg of water. What is the molality of the solution? 0.5 moles = 0.5 molal 1 Kg of H2O
Molality
• Example: What is the molality of a solution prepared by dissolving 8.87 g of methanol CH3OH, in 20.0 g of water?
Calculate the moles of solute (CH3OH) 8.87 g = 0.277 mol 32.04 g
Molality
Calculate the molality m = 0.277 mol CH3OH x 1000 g solvent 20.0 g H2O 1 Kg solvent = 13.8 m
Molality
• Determine the molality of 3000 g of solution containing 37.3 g of KCl. 37.3 g KCl = 0.5 moles KCl 74.6 g
Molality
• Determine the grams of pure solvent. Total = mass of solute + mass of solvent 3000 g = 37.3 g + mass of solvent 3000 g – 37.3 g = 2962.7 g of solvent
Molality
• Convert grams of solvent to kilograms 2962.7 g x 1 Kg = 2.9627 Kg 1000 g • m = 0.5 moles = 0.169 m 2.9627 Kg
Changes in Vapor Pressure
• Nonvolatile solutes always lower the vapor pressure of a solution. • Vapor pressure (VP) is the result of evaporating solvent. • VP depends on temperature. • In a closed container, VP reaches a constant level at any given temperature.
Changes in Vapor Pressure
• Animation showing how VP is established. • Animation showing how VP is affected by temperature. • How does a solute affect VP?
Changes in Vapor Pressure
• Solvent molecules must be at the surface of the liquid to evaporate. • A surface molecule gets bumped from below and is “launched” into the vapor phase.
Solution Vapor