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Chapter 15
The Chemistry of Solutes
and Solutions
William
Henry
1774-1836.
Henry’s Law.
Francois
Raoult
1830-1901.
Raoult’s Law.
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The Solution Process
• Solutes and solvent are components of the solution.
• In the process of making solutions with condensed
phases, intermolecular forces become rearranged.
• Consider NaCl (solute) dissolving in water (solvent):
– the water H-bonds have to be interrupted,
– NaCl dissociates into Na+ and Cl-,
– ion-dipole forces form: Na+ … -OH2 and Cl- … +H2O.
Na+
δ-
O
H
H
Cl-
δ+
H
O
H
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The Solution Process
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The Solution Process
• “Rule”:
LIKE DISSOLVES LIKE
• polar solvents dissolve polar solutes.
• Non-polar solvents dissolve non-polar solutes.
Water is polar (because it’s bent).
It will therefore tend to dissolve other polar molecules or ions.
For example, most salts, alcohols and sugars dissolve in water.
Alcohols and sugars all contain the O-H part of a molecule which
makes them polar:
X
O
H
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The Solution Process
Most organic substances (compounds of carbon) are non-polar.
That is why they in general do not dissolve in water (which is polar).
H
H
H
C
C
C
H
H
C
H
H
H
Generally, carbon chains are non-polar (no dipole moment).
Since the electronegativities of hydrogen and carbon are virtually
the same, hydrocarbons are non-polar. Gasoline is non-polar,
because it is a hydrocarbon.
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Solute-Solvent Interactions
•
•
•
•
Miscible liquids: mix in any proportions.
Immiscible liquids: do not mix.
Soluble liquids: mix in certain proportions.
Intermolecular forces are important: water and
ethanol are miscible because the broken hydrogen
bonds in both pure liquids are re-established in the
mixture.
• The number of carbon atoms in a chain affect
solubility: the more C atoms the less soluble in water.
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Factors Affecting Solubility
Solute-Solvent Interactions
ethanol ---- ethanol
ethanol ---- water
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Factors Affecting Solubility
Solute-Solvent Interactions
• The number of -OH groups within a molecule
increases solubility in water.
Glucose – (a sugar)
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Ways of Expressing Concentration
• All methods involve quantifying amount of solute per
amount of solvent or solution.
• Amounts are masses, moles or liters.
• Qualitatively solutions are dilute or concentrated.
• Definitions:
Mass of component = mass of component in soln
fraction
total mass of soln
ppm of component =
mass of component in soln
 10 6
total mass of soln
Mass % = mass fraction  100
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Ways of Expressing Concentration
Mole Fraction, Molarity, and Molality
Mole fraction of solute =
X
moles of solute
total moles of all components
moles of solute
Molarity =
liters of solution
M
moles of solute
=
Molality
kg of solvent
m
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The Composition of the Oceans
Concentrations in ppm and Molarity
Chloride
19350 0.55
Sodium
10760
0.47
Sulfate
2710
0.028
Magnesium
1290
0.054
Calcium
412 0.010
Potassium
400 0.010
Carbon dioxide* 106 2.3 x 10-4
Bromide
67
8.3 x 10-4
Boric acid
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4.3 x 10-4
Strontium
7.9 9.1 x 10-5
Fluoride
1.3
7.0 x 10-5
* present as bicarbonate and carbonate
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Factors Affecting Solubility
Pressure Effects: solubility of a gas in a liquid is a
function of the pressure of the gas as given by:
Henry’s Law:
C g = kPg
Cg is the solubility of gas,
Pg the partial pressure,
k = Henry’s law constant.
•Carbonated beverages are bottled under PCO2 > 1 atm.
As the bottle is opened, PCO2 decreases and the solubility
of CO2 decreases. Therefore, bubbles of CO2 escape from
solution.
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Temperature effects
dissolving gases
opening pop bottles
thermal pollution in lakes
dissolving solids
sugar dissolves better in warm
water
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Lowering the Vapor Pressure
•Non-volatile solutes 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 (this is a colligative property)
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Colligative Properties
• Colligative properties depend on number of
(nonvolatile) solute molecules.
• Some colligative properties:
–Vapor pressure lowering
–Boiling point elevation
–Freezing point depression
–Osmotic pressure
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Colligative Properties
Raoult’s Law (used for vapor pressure lowering)
• PA is the vapor pressure of A in a solution
• PA is the vapor pressure of pure A
• A is the mole fraction of A in solution,
PA =  A PA 
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Colligative Properties
Boiling-Point Elevation
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Colligative Properties
Boiling-Point Elevation
• At 1 atm (normal boiling point of pure liquid) there is
a lower vapor pressure of the solution. Therefore, a
higher temperature is required to reach a vapor
pressure of 1 atm for the solution (Tb).
• Molal boiling-point-elevation constant, Kb, expresses
how much Tb changes with molality, m:
Tb = K b m
Kb depends only on solvent
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Colligative Properties
Freezing-Point Depression
• The solution freezes at a lower temperature (Tf) than
the pure solvent.
• Decrease in freezing point (Tf) is directly
proportional to molality (Kf is the molal freezingpoint-depression constant):
T f = K f m
Kf depends only on solvent
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ΔTb =Kbm
ΔTf=Kfm
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Colligative Properties
Can be used to determine molecular mass of solute.
Problem: Soln of 5.00 g lauryl alcohol (solute) dissolved in 100 g
benzene (solvent) freezes at 4.1oC. Calculate molar mass of lauryl
alcohol.
Use ΔTf= Kfm;
ΔTf = 4.1oC
ΔTf=5.5-4.1=1.4oC
Kf = 5.12oC/m, fp = 5.5° for benzene
m= ΔTf/Kf = 1.4oC/(5.12oC/m) = 0.27 m
molality is mol solute/kg solvent
0.27mol solute
x0.100kg benzene = 0.027mol solute
kg benzene
5.00g
MolarMass solute =
= 183g/mol
0.027mol
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Semipermeable membrane
more concentrated
less concentrated
solution
solution
OSMOSIS
H2O
H2O
Particles of solute
Particles of solute
H2O
H2O
Water
more concentrated
less concentrated
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Osmotic pressure
causes a difference
in levels
of water
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V = nRT
n
 =   RT
V 
= MRT
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Colligative Properties
Osmosis
• Osmotic pressure, , is the pressure required to stop
osmosis:
• Isotonic solutions: two solutions with the same 
separated by a semipermeable membrane.
• Hypotonic solutions: a solution of lower  (less
concentrated) than a hypertonic solution (more
concentrated).
• Red blood cell walls are semipermeable membranes.
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Colligative Properties
Osmosis
• Crenation and Hemolysis:
(Cell in hypertonic solution)
Crenation (shrivels up)
Salty food causes retention
of water and swelling
of tissues (edema)
(Cell in hypotonic solution)
Hemolysis (bursts)
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TOTAL NUMBER OF PARTICLES
Colligative properties depend on total numbers of particles, and
salts give more than 1 mole of particles per mole of compound.
A 1.0 m solution of NaCl provides 1.0 mol of Na+ and 1.0 mol of ClThus, 1.0 mol of NaCl provides 2.0 mol of ions (particles)
The colligative property is enhanced by a factor of 2.
Other examples:
1.0 mol of Na2SO4 provides 3.0 mol of ions.
1.0 mol of (NH4)3PO4 provides 4.0 mol of ions.
Covalent molecules like sucrose or ethylene glycol do not ionize.
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Colloids
Colloids are huge molecules which create suspensions with
properties between those of mixtures and solutions.
Examples: smoke, fog, mayonnaise, marshmallows, emulsions,
colored glass.
The Tyndall effect is used to characterize liquid colloids in water: a
beam of light through a colloidal suspension can be seen from the
side (because of the scattered light); a beam of light through a true
solution is not visible from the side.
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