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III. Colligative Properties

Solutions. III. Colligative Properties. A. Definition. Colligative Property property that depends on the number (amount) of solute particles in the solution & not in the nature of the solute particles. Colligative Properties. Vapor Pressure Lowering Boiling Point Elevation

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III. Colligative Properties

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  1. Solutions III. Colligative Properties

  2. A. Definition • Colligative Property • property that depends on the number (amount) of solute particles in the solution & not in the nature of the solute particles

  3. Colligative Properties • Vapor Pressure Lowering • Boiling Point Elevation • Freezing Point Depression • Osmotic Pressure

  4. Vapor Pressure Lowering • Vapor Pressure- pressure exerted by the vapor/gas above the liquid • VP of sol’n < VP of solvent P = P1º-P1 = X2P1º where: P1º = V.P. of solvent X2 = mole frxn of solute P1 = V.P. of solution

  5. Boiling Point Elevation • Boiling Point- the temperature at which the vapor pressure equals the external atmospheric pressure. • B.P. of sol’n > B.P. of solvent Tb= Tb-Tbº = i Kbm Tbº = B.P. of solvent Kb = molal B.P. constant Tb = B.P. of solution = (0.52 °C/molal for H2O) m = molality of sol’n i = van’t Hoff Factor

  6. van’t Hoff Factor, i i = actual # of particles in sol’n after dissociation # of formula units initially dissolved in sol’n • Nonelectrolytes (covalent) • remain intact when dissolved • 1 particle • Electrolytes (ionic) • dissociate into ions when dissolved • 2 or more particles

  7. Boiling Point Elevation Solute particles weaken IMF in the solvent.

  8. Applications - Fractional Distillation

  9. Freezing Point Depression • Freezing Point- the temperature at w/c a liquid phase is converted to the solid phase • F.P. of sol’n < F.P. of solvent Tf= Tfº - Tf = i Kfm Tfº = F.P. of solvent Kf = molal F.P. constant Tf = F.P. of solution = (1.86 °C/molal for H2O) m = molality of sol’n i = van’t Hoff Factor

  10. Applications • salting icy roads • making ice cream • antifreeze • cars (-64°C to 136°C) • fish & insects

  11. Osmotic Pressure • Osmosis- the selective passage of solvent molecules from a semi-permeable membrane from a dilute solution to more concentrated one. • Osmotic Pressure- pressure needed to prevent osmosis.  = i MRT  = osmotic pressure T = absolute temp M = molarity of sol’n i = van’t Hoff Factor R = 0.082058 L-atm/mol-K

  12. Isotonic sol’n – sol’ns w/ same osmotic pressure / con’c • Hypotonic sol’n – the sol’n w/ lower con’c & osmotic pressure • Hypertonic- the sol’n w/ higher con’c & osmotic pressure

  13. Applications - Reverse Osmosis (Desalination of Water)

  14. NOTES • The effects of Colligative Properties on ELECTROLYTE soln’s are GREATER compared to NON_ELECTROLYTE soln’s.

  15. Calculations 1. What are the B.P. & F.P. of a 2.47 molal sol’n of naphthalene in benzene, (naphthalene does not ionize in benzene), given the ff data for benzene: BP = 80.1 °C Kb = 2.53 °C/molal FP = 5.5 °C Kf = 5.12 °C/molal

  16. Calculations 2. A sol’n containing 0.8330 g of a non-dissociating polymer of unknown structure in 170.0 mL of an organic solvent was found to have an osmotic pressure of 5.30 mmHg at 25°C. Determine the molar mass of the polymer. 3. The freezing point depression of a 0.100 M MgSO4(aq) sol’n is 0.225 °C. Calculate the van’t Hoff Factor of MgSO4(aq) at this con’c.

  17. Calculations 4. How many grams of Urea [(NH2)2CO] must be added to 450 g of H2O to give a sol’n w/ a V.P. of 2.50 mmHg less than that of pure H2O at 30°C. (V.P. of H2O at 30°C is 31.8 mmHg) 5. The average osmotic pressure of seawater is about 30.0 atm at 25°C. Calculate the molar con’c of an aqueous sol’n of Urea [(NH2)2CO], that is isotonic w/ seawater.

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