II III I IV. Colligative Properties of Solutions Ch. 16 – Mixtures & Solutions

A. Definition Colligative Property Colligative Property property that depends on the number of solute particles, not their identity in an ideal solution

B. Types Freezing Point Depression Freezing Point Depression ( T f ) f.p. of a solution is lower than f.p. of the pure solvent Boiling Point Elevation Boiling Point Elevation ( T b ) b.p. of a solution is higher than b.p. of the pure solvent

B. Types View Flash animationFlash animation Freezing Point Depression

B. Types Solute particles weaken IMF in the solvent Boiling Point Elevation

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

C. Calculations t :change in temperature (° C ) k :constant based on the solvent (° C·kg/mol ) m :molality ( m ) n :# of particles t = k · m · n

C. Calculations T : change in temperature (° C ) i : Vant Hoff Factor (VHF), the number of particles into which the solute dissociates m : molality ( m ) K : constant based on the solvent (° C·kg/mol ) or (°C/ m ) T = i · m · K

C. Calculations # of Particles # of Particles Nonelectrolytes (covalent) remain intact when dissolved 1 particle Electrolytes (ionic) dissociate into ions when dissolved 2 or more particles

C. Calculations At what temperature will a solution that is composed of 0.73 moles of glucose in 225 g of phenol boil? WORK: GIVEN:

C. Calculations At what temperature will a solution that is composed of 0.73 moles of glucose in 225 g of phenol boil? m = 3.2m n = 1 t b = k b · m · n WORK: m = 0.73mol ÷ 0.225kg GIVEN: b.p. = ? t b = ? k b = 3.04°C·kg/mol t b = (3.04°C·kg/mol)(3.2m)(1) t b = 9.7°C b.p. = 181.8°C + 9.7°C b.p. = 192°C

C. Calculations Find the freezing point of a saturated solution of NaCl containing 28 g NaCl in 100. mL water. WORK: GIVEN:

C. Calculations Find the freezing point of a saturated solution of NaCl containing 28 g NaCl in 100. mL water. m = 4.8m n = 2 t f = k f · m · n WORK: m = 0.48mol ÷ 0.100kg GIVEN: f.p. = ? t f = ? k f = 1.86°C·kg/mol t f = (1.86°C·kg/mol)(4.8m)(2) t f = 18°C f.p. = 0.00°C - 18°C f.p. = -18°C

C. Calculations T Change in temperature Not actual freezing point or boiling point Change from FP or BP of pure solvent Freezing Point (FP) i T F is always subtracted from FP of pure solvent Boiling Point (BP) i T B is always added to BP of pure solvent

C. Calculations i – VHF i – VHF Nonelectrolytes (covalent) remain intact when dissolved 1 particle Electrolytes (ionic) dissociate into ions when dissolved number of ions per formula unit 2 or more particles

C. Calculations i – VHF i – VHF Examples CaCl 2 Ethanol C 2 H 5 OH Al 2 (SO 4 ) 3 Methane CH 4 i =

C. Calculations K – molal constant K – molal constant K F K F – molal freezing point constant Changes for every solvent 1.86 °C·kg/mol (or °C/ m ) for water K B K B – molal boiling point constant Changes for every solvent °C·kg/mol (or °C/ m ) for water

C. Calculations: Recap! : subtract from F.P. T : subtract from F.P. add to B.P. add to B.P. i – VHF : covalent = 1 i – VHF : covalent = 1 ionic > 2 ionic > 2 K : K F water = K : K F water = 1.86 °C·kg/mol K B water = K B water = °C·kg/mol T = i · m · K

At what temperature will a solution that is composed of moles of glucose in 225 g of water boil? C. Calculations m = 3.24 m K B = 0.512°C/ m T B = i · m · K B WORK: m = mol ÷ kg GIVEN: b.p. = ? T B = ? i = 1 T B = (1)(3.24 m )(0.512°C/ m ) T B = 1.66°C b.p. = °C °C b.p. = °C T b

C. Calculations Find the freezing point of a saturated solution of NaCl containing 28 g NaCl in 100. mL water. i = 2 m = 4.8 m K F = 1.86°C/ m T F = i · m · K F WORK: m = 0.48mol ÷ 0.100kg GIVEN: f.p. = ? T F = ? T F = (2)(4.8 m )(1.86°C/ m ) T F = 18°C f.p. = 0.00°C – 18°C f.p. = -18°C 0 – T F

D. Osmotic Pressure Osmosis: The flow of solvent into a solution through a semipermeable membrane Semipermeable Membrane: membrane that allows solvent to pass through but not solute

D. Osmotic Pressure Net transfer of solvent molecules into the solution until the hydrostatic pressure equalizes the solvent flow in both directions

Because the liquid level for the solution is higher, there is greater hydrostatic pressure on the solution than on the pure solvent Osmotic Pressure: The excess hydrostatic pressure on the solution compared to the pure solvent D. Osmotic Pressure

Osmotic Pressure: Minimum Pressure required to stop flow of solvent into the solution D. Osmotic Pressure

Osmosis at Equilibrium

= i M R T where: osmotic pressure (atm) π = osmotic pressure (atm) VHF i = VHF M = Molarity (moles/L) R = Gas Law Constant T = Temperature (Kelvin) E. Osmotic Pressure Calculations L atm/mol K

E. Osmotic Pressure Calculations Calculate the osmotic pressure (in torr) at 25 o C of aqueous solution containing 1.0g/L of a protein with a molar mass of 9.0 x 10 4 g/mol. i = 1 M = 1.11 x M R = L atm/mol K T = 25 o C = 298 K WORK: M = 1.0 g prot. GIVEN: = ? 1.11 x M = (1)(1.11x10 -5 )(.08206)(298) = x atm = 0.21 torr 1 mol prot. 1 L soln 9.0 x 10 4 g =

Colligative Properties useful for: characterizing the nature of a solute after it is dissolved in a solvent determining molar masses of substances D. Osmotic Pressure

If the external pressure is larger than the osmotic pressure, reverse osmosis occurs One application is desalination of seawater F. Reverse Osmosis

Net flow of solvent from the solution to the solvent

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