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Colligative Properties

Colligative Properties. Colligative Properties. Colligative properties depend on quantity of solute molecules. Vapor pressure lowering Boiling point elevation Freezing point depression Osmotic pressure. Colligative Properties of Solutions. Definition:

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Colligative Properties

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  1. Colligative Properties

  2. Colligative Properties • Colligative properties depend on quantity of solute molecules. • Vapor pressure lowering • Boiling point elevation • Freezing point depression • Osmotic pressure

  3. Colligative Properties of Solutions Definition: Properties, that depend on the NUMBER of solute particles present in solution Concentration: Osmol per liter or osmolarity An OSMOL is a mole of solute particle

  4. doesnot dissociate C6H12O6 C6H12O6 1 osmol/L 1 M (6 x 1023) doesdissociate NaCl Na+ + Cl- 1 osmol/L 1 osmol/L 1 M 2 x (6 x 1023) 2 osmol/L

  5. solvent solution Lowering the Vapor Pressure • Non-volatile solvents reduce the ability of the surface solvent molecules to escape the liquid. • Therefore, vapor pressure is lowered. • The amount of vapor pressure lowering depends on the amount of solute.

  6. Vapor pressure lowering • Raoult’s Law – a nonvolatile solute will lower the vapor pressure of a solvent • Pure water will have a higher vapor pressure than salt water. • Ideal solution: one that obeys Raoult’s law. • Raoult’s law breaks down when the solvent-solvent and solute-solute intermolecular forces are greater than solute-solvent intermolecular forces.

  7. 760 torr Liquid Solid Pressure Gas ΔTm ΔTb Temperature Phase Diagram Analysis

  8. Boiling-Point Elevation Interpret the phase diagram for a solution. Non-volatile solute lowers the vapor pressure. At 1 atm (normal bp of pure liquid) there is a lower vapor pressure of the solution.  a higher bp is required to reach a vapor pressure of 1 atm for the solution. ΔTb = Kbmi

  9. Freezing-Point Depression

  10. Freezing-Point Depression • At 1 atm (normal boiling point of pure liquid) there is no depression by definition • The solution freezes at a lower temperature (ΔTf) than the pure solvent. • Decrease in freezing point (ΔTf) is directly proportional to molality (Kf is the molal freezing-point-depression constant): • ΔTf = Kfmi • Colligative properties can be used to determine the MW of an unknown compound.

  11. Raoult’s law Psoln= vapor pressure of solution Psoln = Xsolv P0 Xsolv= mole fraction of solvent P0= vapor pressure of pure solvent DTf = Kf m DTf= freezing point depression Kf= cryoscopic constant m= molality 1. Vapor pressure lowering 2. Freezing point depression

  12. DT= boiling point elevation DTb = Kb m Kb= ebullioscopic constant m= molality 3. Boiling point elevation Cryoscopic and ebullioscopic constants

  13. p = M  RT p = osmotic pressure M = molarity R = 8.314 JK-1 mol-1 4. Osmotic pressure

  14. Dilute solution Concentrated solution Membrane Osmotic Pressure • Semipermeable membrane: permits passage of some components of a solution. Example: cell membranes and cellophane. • Osmosis: the movement of a solvent from low solute concentration to high solute concentration. • There is movement in both directions across a semipermeable membrane. • As solvent moves across the membrane, the fluid levels in the arms becomes uneven. • Eventually the pressure difference between the arms stops osmosis.

  15. Osmotic Pressure • Osmotic pressure, P, is the pressure required to stop osmosis. • Isotonic solutions: two solutions with the same P separated by a semipermeable membrane. • Hypotonic solutions: a solution of lower P than a hypertonic solution. • Osmosis is spontaneous.

  16. Osmotic Pressure & Biology • Red blood cells are surrounded by semipermeable membranes. • Crenation: • red blood cells placed in hypertonic solution (a lower solute concentration exists in the cell) • osmosis occurs and water passes out of the cell causing the cell to shrivel up. • Hemolysis is opposite • red blood cells placed in a hypotonic solution (a higher solute concentration in the cell) • water moves into the cell causing the cell to burst. • To prevent crenation or hemolysis, IV (intravenous) solutions must be isotonic.

  17. P > P Drinking water is produced by applying a pressure that exceeds P. Osmotic Pressure Applications • Pickling food by placing in salty solutions. • Water moves into plants through osmosis. • Salt added to meat or sugar to fruit prevents bacterial infection (a bacterium placed on the salt will lose water through osmosis and die). • Dialysis machines work by osmosis. • Reverse osmosis is used in desert countries to produce drinking water from the sea. Salty sea water would normally draw in more water to this side

  18. Electrolytes ‘Anomalous’ behavior: • Ability to conduct electric current • Greater effect on colligative properties DTf values: (kf = 1.86°C for H2O)

  19. measured value expected value i = DTf Kfm i = van’t Hoff factor: i DTf = Kfm

  20. DTf = i Kfm DTb = i Kbm p = i MRT i values:

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