colligative properties vapor pressure lowering l the bonds between molecules keep molecules of a...
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Colligative
Properties
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Vapor Pressure Lowering
The bonds between molecules keep molecules of a liquid from escaping into the vapor state.
In a solution, some of the solvent is busy keeping the solute dissolved.
This lowers the vapor pressure.
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Vapor Pressure Lowering
Electrolytes form ions when dissolved.
NaCl Na+ + Cl- 2 ionsCaCl2 Ca+2 + 2Cl- 3 ions Calcium chloride would lower the
vapor pressure more because it dissociates into more ions.
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Boiling Point Elevation
The vapor pressure determines the boiling point.
The lower the vapor pressure, the higher boiling point.
It turns out that the boiling point of a solution is higher than the boiling point of the pure solvent.
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Boiling Point Elevation
Salt water boils above 100ºC.KBr K+ + Br- 2 ionsMgF2 Mg+2 + 2F- 3 ions Magnesium fluoride would raise the
boiling point more because it dissociates into more ions.
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Freezing Point Depression
Solids form when molecules make an orderly pattern.
The solute molecules break up the orderly pattern.
This makes the freezing point lower.
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Freezing Point Depression
Salt water freezes below 0ºC.K2SO4 2K+ + SO4
-2 3 ionsNaCl Na+1 + Cl- 2 ions Potassium sulfate would lower the
freezing point more because it dissociates into more ions.
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MolalityMolality is another unit for
concentration.
m =
1 kg = 1000 g
moles of solute
kilogram of solvent
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Example
What is the molality of a solution with 9.3 mole of NaCl in 450 g of water?
moles of solute
kilogram of solventm =
0.450 kg
9.3 mol
m = 21 m
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Problem
What is the molality of a solution with 11.3 mole of KBr in 650. g of water?
(17.4 m)
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Problem
What is the molality of a solution with 142 g of KBr in 750. g of water?
(1.59 m)
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Why Molality?
The size of the change in boiling point is determined by the molality.
Tb = Kb x m x nTb is the change in the boiling pointKb is a constant determined by the
solventm is the molality of the solution.n is the number of ions the solute
falls into when it dissolves.
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What about Freezing?The size of the change in freezing
point is determined by the molality.Tf = -Kf x m x nTf is the change in the boiling pointKf is a constant determined by the
solvent.m is the molality of the solution.n is the number of ions the solute
falls into when it dissolves.
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Example What is the boiling point of a solution
made by dissolving 1.20 moles of NaCl in 750. g of water? Kb for water is 0.51.
moles of solute
kilogram of solventm =
0.750 kg
1.20 mol
m = 1.60 m
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Example What is the boiling point of a solution
made by dissolving 1.20 moles of NaCl in 750. g of water? Kb for water is 0.51.
KbmΔTb = (1.60)0.51
ΔTb = 1.63 °C
n(2)
So Tb = 100 + 1.63 = 101.63 °C
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Example What is the freezing point of a solution
made by dissolving 1.45 moles of CaCl2 in 850. g of water? Kf for water is 1.86.
moles of solute
kilogram of solventm =
0.850 kg
1.45 mol
m = 1.71 m
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Example What is the freezing point of a solution
made by dissolving 1.45 moles of CaCl2 in 850. g of water? Kf for water is 1.86.
- KfmΔTf = (1.71)1.86
ΔTf = - 9.54 °C
n(3)
So Tf = 0 + - 9.54 = - 9.54 °C
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ColloidsSometimes, mixtures are partway
between true solutions and heterogeneous mixtures.
Such mixtures, called colloids, contain particles that are evenly distributed through a dispersing medium, and remain distributed over time rather than settling out.
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ColloidsFamiliar colloids include fog, smoke,
homogenized milk, and ruby-colored glass.
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ColloidsA beam of light can be used to
distinguish a colloidal suspension from a true solution. This is referred to as the Tyndall Effect.
The Tyndall Effect is caused by reflection of light by very small particles in suspension in a trasnparent medium.
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ColloidsA beam of light can be used to
distinguish a colloidal suspension from a true solution. This is referred to as the Tyndall Effect.
The Tyndall Effect is caused by reflection of light by very small particles in suspension in a trasnparent medium.
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ColloidsThe Tyndall effect is seen below
using a laser pointer. The glass on the left contains colloidal silver and the one on the right is water from the tap after the bubbles have settled out.