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Colligative Properties

Colligative Properties. Physical properties affected by dissolved solute particles Type of solute doesn’t matter. Number of particles does matter. Colligative means “depending on the collection”. Colligative Properties.

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Colligative Properties

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  1. Colligative Properties • Physical properties affected by dissolved solute particles • Type of solute doesn’t matter. • Number of particles does matter. • Colligative means “depending on the collection”

  2. Colligative Properties • Occur with nonvolatile solutes – ones that have little tendency to become a gas • 4 colligative properties: • Vapor pressure lowering • Boiling point elevation • Freezing point depression • Osmotic pressure

  3. Colligative Properties • Electrolytes are molecules that ionize or fall apart when in solution; nonelectrolytes do not fall apart or ionize • Electrolytes will have a greater impact on the 4 colligative properties • Why? • Sugar (C12H22O12) remains as one molecule when dissolved. • Salts like MgCl2 dissociate in water so one mole of MgCl2 actually forms 3 moles of ions.

  4. Vapor Pressure Lowering • Pressure in a CLOSED container exerted by the particles of liquid that have escaped and become gas particles • Greater number of escaped particles = greater vapor pressure • The addition of a solute LOWERS the vapor pressure – WHY? • The solute molecules get in the way of the solvent molecules trying to escape at the surface of the liquid.

  5. Boiling Point Elevation • A liquid boils when it’s particles can escape as a gas (or when the vapor pressure is equal to the atmospheric pressure) • Ex: salt in water when cooking pasta antifreeze in radiators • When a solute is added, the boiling point is driven higher – WHY? • The solute molecules get in the way of the solvent molecules trying to escape at the surface of the liquid.

  6. Freezing Point Depression • A liquid freezes when its particles do not have enough energy to overcome the intermolecular forces between them. • So the molecules are pulled closer together and take on a defined shape. • When a solute is added, the freezing point is driven lower – WHY? • The solute molecules get in the way and make it harder for the solvent molecules to move closer and become a solid • Ex: salt & sand on icy roads ethylene glycol on icy airplanes

  7. Calculating b.p. elevation and f.p. depression • Because the type of solute doesn’t matter, you only need to know the amount of solute • Elevation & depression happen at a constant rate depending on the amount of solute added (number of moles) • Tb = Kbm (where Kb & Kf are the constants) • Tf = Kfm

  8. Try It • If you have .625 m aqueous solution with a nonvolatile, nonelectrolyte solute, what would the boiling point and freezing point be?

  9. Try It • Calculate the freezing point and boiling point of a solution that contains 105.4 g NaCl dissolved in 750.3 ml H2O.

  10. Osmotic Pressure • Osmosis is a natural process where the solvent flows across a membrane – always flows from where there is more solvent to where there is less solvent. • Adding solute to one side of the membrane will cause more solvent to flow that way. • This increases the osmotic pressure on that side of the membrane. • Ex: kidney dialysis uptake of nutrients by cells

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