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Colligative Properties of Electrolytes

Since colligative properties depend on the number of

particles dissolved, solutions of electrolytes (which

dissociate in solution) should show greater changesthan those of nonelectrolytes.

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Colligative Properties of Electrolytes

However, a 1 M solution of NaCl does not showtwice the change in freezing point that a 1 Msolution of methanol does.

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Ion Pairing and the vant Hoff FactorOne mole of NaCl in water doesnot really give rise to two moles ofions.

Ion pairing and colligativeproperties. Ion pairing

becomes more prevalent as

the solution concentrationincreases

Some Na+

and Cl

reassociate

for a short time, so the true

concentration of particles is

somewhat less than two timesthe concentration of NaCl.

Reassociation is more likely at

higher concentration.

Therefore, the number of

particles present is

concentration dependent.

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The vant Hoff FactorWe modify theprevious equations by

multiplying by the

vant Hoff factor, i= lyte)nonelectroforcalculated(Tmeasured)(T ff

lyte)nonelectroforcalculated(T

measured)(T

f

f

=i

For an ideal solution: the vant Hoff factor equals the number of ionsper formula unit.For ideal solutions of NaCl

and K2

SO4 : i = 2 and i = 3, respectively

In absence of information about the value of i for a solution, usethe ideal value in calculations

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Osmosis

Some substances form semipermeable membranes (e.g

cellophane and many membranes in biological systems),allowing some smaller particles to pass through (e.g.

water molecules), but blocking other larger particles.

In biological systems, most semipermeable membranesallow water to pass through, but solutes are not free to do

so.

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Osmosis

In osmosis, there is net movement of solvent from the area ofhighersolvent concentration (lower solute concentration) to the are oflower solvent concentration (higher solute concentration). There isa net solvent movement through the semipermeable membrane, as ifthe solutions were driven to attain equal concentrations acrossthe membrane

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Osmotic PressureThe pressure required to stop osmosis, known as osmotic

pressure, , isnRTV =

If the osmotic pressure is the same on both sides of a membrane (i.e., the

concentrations are the same), the solutions are

isotonic. If one solution is

of lower osmotic pressure, it is hypotonic with respect to the more

concentrated solution. The more concentrated solution is hypertonic with

respect to the dilute solution.

V is the volume of the solution, n is the number of moles of

the solute, R is the gas constant, and T is the temperature in

Kelvin

MRTRTVn =

=Then

M is the molarity of the solution (mol/L)

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Osmosis in Blood Cells

If the solute concentration

outside the cell is hypertonicrelative to the intracellular

solution (the solution within

the cell)

Water will flow out of the

cell, this cause the cell toshrivel, a process called

crenation

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Osmosis in Cells

If the soluteconcentration outside thecell is hypotonic relativeto the intracellularsolution.

Water will flow into thecell. This causes the cellto rupture, a processcalled hemolysis.

The Intravenous (IV) solutions must be isotonic with

the intracellular fluids of the cell otherwise crenation

orhemolysis occur

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A solution of an unknown nonvolatile electrolyte was prepared by dissolving0.250 g of the substance in 40.0 g of CCl4. The boiling point of the resultantsolution was 0.357C higher than that of the pure solvent. Calculate the molarmass of the solute. For pure CCl4 kb

= 5.02 C/m

EXERCISE: Molar Mass from Freezing-Point Depression

m0711.0

C.m5.02

C375.0

k

T

kg1040.0

molnmolality

1-b

b3-

solute ==

=

=D

D

Answer:

soluteofmol102.84)(mol.kg0711.0kg1040.0n

m0711.0kg1040.0

n

3-1-3-solute

3-solute

==

=

1-3-

solute

3-solute

g.mol0.88mol102.84

g250.0(mol)n

(g)mMM

soluteofmol102.84MM

mn

=

==

==

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The osmotic pressure of an aqueous solution of a certain protein was measured

in order to determine the proteins molar mass. The solution contained 3.50 mgof protein dissolved in sufficient water to form 5.00 mL of solution. Theosmotic pressure of the solution at 25C was found to be 1.54 torr. Calculatethe molar mass of the protein.

EXERCISE: Molar Mass from Osmotic Pressure

nRTV =Answer

mol1014.4ml1000

L1ml5

mmHg760

atm1

K298.KL.atm.mol0.0821

mmHg54.1

RTVn

71-1-

=

=

=

1-37-

3

g.mol1045.8

mol104.14

g1050.3

n

mMM

MM

mn =

===

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ColloidsSuspensions or dispersions of particles larger than individual

ions or molecules, but too small to be settled out by gravity.

Colloids form the dividing line between solutions and

homogeneous mixtures

Size of colloid particles range from 5 to 1000 nm

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Tyndall Effect

Colloidal suspensions

can scatter rays of light. This phenomenon isknown as the Tyndall

effect.

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Colloids in Biological Systems

Some molecules havea polar, hydrophilic (water-loving) end anda nonpolar,hydrophobic (water-hating) end.