Chapter 11
Solutions and Their Properties
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I. Introduction
A) Solution - A homogeneous mixture of two or more substances in a single
phase (gas, liquid, solid)
• Alloy - A solid solution (i.e. brass is a solution of Cu and Zn)
• Solvent • Solute -
The component that makes up the bulk of the solution.
The component that dissolves in the solvent.
B) Colligatve Properties -
Properties of solutions which depend on the
number of solute particles in the solution and not
the nature of the solute.
• Four Colligative Properties
1)
2)
3)
4)
Vapor pressure lowering
Boiling point elevation
Freezing point depression
Osmosis
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II. Units of Concentration
moles solute (mol)
Molarity (M) 
liters solution (L)
• Not useful in colligative properties because the exact amount of
solvent is unknown.
• The following concentration units reflect the number of solute
particles per solvent molecules and are useful with colligative
properties.
Molality (m) 
moles solute (mol)
kilograms of solvent
Weight % of A 
Mole Fraction (X A ) 
nA
nA  nB  nC  
mass A
 100
mass A  mass B  mass C    
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II. Units of Concentration
• Naturally occurring solutions are often very dilute so
environmental chemists, biologists, geologists, etc. often use
parts per million (ppm).
1 mg
1 mg
Parts per million (ppm) 
or
1 kg
1L
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IV. Colligative Properties
• Colligative properties are independent of the nature of the
solvent and depend only on the relative number of solute and
solvent particles.
•
There are four colligative properties of solutions:
A.
B.
C.
D.
Vapor Pressure Lowering
Boiling Point Elevation
Freezing Point Depression
Osmosis
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IV. Colligative Properties
A. Vapor Pressure Lowering
•
The vapor pressure of the solution is lowered because the
solute particles at the liquid/vapor boundary block the
solvent particle from jumping into the vapor state.
Raoult’s Law
Psolv = XsolvP°solv
or
DPsolv = XsoluteP°solv
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IV. Colligative Properties
B. Boiling Point Elevation
• For nonvolatile, nonelectrolyte solvents the change in
boiling point (DTbp) is:
DTbp = Kbpmsolute
Kbp = boiling point elevation
constant
msolute = molality of solute
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IV. Colligative Properties
C. Freezing Point Depression
• For nonvolatile, nonelectrolyte solvents the change in
freezing point (DTfp) is:
DTfp = Kfpmsolute
Kfp = freezing point depression
constant
msolute = molality of solute
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IV. Colligative Properties
D. Colligative Properties and Molar Mass Determination
• Colligative properties can be used to determine the molar
mass of a solute when it is dissolved in a solvent of
appreciable vapor pressure and a known Kbp or Kfp.
Measure a
change in VP,
BP elevation,
FP depression,
or osmotic
pressure.
Solution
Conc.
Use mass
of solvent
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Moles of
Solute
g solute
mol solute
Molar
Mass
IV. Colligative Properties
E. Colligative Properties of Solutions Containing Ions
•
The change in VP, BP, FP, or osmotic pressure is greater than expected for electrolyte (ionic
salt) solutions.
Predicted BP elevation of an aqueous 0.100 m NaCl solution
DTbp = Kbp • msolute (For H2O Kbp = 0.5121 °C/m)
DTbp, calculated = (0.5121 °C/m)(0.100 m) = 0.05121
°C Actual BP elevation of an aqueous 0.100 m NaCl solution
DTbp, measured = 0.09470 °C
•
(Almost double the DTbp calculated)
Colligative properties depend on the total number of solute particles in solution. Ionic
compounds form ions in solution so the total number of solute particles in solution is equal
to the total ions in solution.
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IV. Colligative Properties
E. Colligative Properties of Solutions Containing Ions
NaCl(s)  Na+(aq) + Cl-(aq)
0.100 m
0.100 m
0.100 m
0.200 m total
van’t Hoff factor (i)
i
DTbp , measured
DTbp ,calculated

0.09470 C
0.05121 C
 1.85
So for ionic solutions:
DTbp  i K bp m
DT fp  i K fp m
Psolvent  i X solvent P

solvent
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IV. Colligative Properties
E. Colligative Properties of Solutions Containing Ions
Predicting van’t Hoff factors
NaCl(s)  Na+(aq) + Cl-(aq)
1 particle + 1 particle = 2 particles
ipredicted = 2
Na2SO4(s)  2 Na+(aq) + SO42-(aq)
2 particles
ipredicted = 3
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+ 1 particle = 3 particles
IV. Colligative Properties
F. Osmosis and Osmotic Pressure
Osmosis - The movement of solvent molecules through a
semipermeable membrane from a region of low
solute concentration to a region of high solute
concentration.
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IV. Colligative Properties
F. Osmosis and Osmotic Pressure
Osmotic Pressure - The pressure created by a column of
solution for a system in equilibrium.
Π  MRT
P  osmotic pressure
M = molar conc. (mol/L)
L atm
R = 0.08206 mol K
T = Temperature (K)
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IV. Colligative Properties
F. Osmosis and Osmotic Pressure
Reverse Osmosis is Used for Water Purification
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Solutions
• Solutions are homogeneous mixtures of two or
more pure substances.
• In a solution, the solute is dispersed uniformly
throughout the solvent.
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Solutions
How does a solid dissolve
into a liquid?
What ‘drives’ the
dissolution process?
What are the energetics of
dissolution?
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How Does a Solution Form?
1. Solvent molecules attracted to surface ions.
2. Each ion is surrounded by solvent molecules.
3. Enthalpy (DH) changes with each interaction broken or
formed.
Ionic solid dissolving in water
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How Does a Solution Form?
1. Solvent molecules attracted to surface ions.
2. Each ion is surrounded by solvent molecules.
3. Enthalpy (DH) changes with each interaction broken or
formed.
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How Does a Solution Form
The ions are solvated
(surrounded by
solvent).
If the solvent is water,
the ions are
hydrated.
The intermolecular
force here is iondipole.
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Dissolution vs reaction
Ni(s) + HCl(aq)
NiCl2(aq) + H2(g)
dry
NiCl2(s)
• Dissolution is a physical change—you can get back the
original solute by evaporating the solvent.
• If you can’t, the substance didn’t dissolve, it reacted.
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Degree of saturation
• Saturated solution
 Solvent holds as much
solute as is possible at
that temperature.
 Undissolved solid
remains in flask.
 Dissolved solute is in
dynamic equilibrium
with solid solute
particles.
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Degree of saturation
• Unsaturated Solution
 Less than the
maximum amount of
solute for that
temperature is
dissolved in the
solvent.
 No solid remains in
flask.
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Degree of saturation
• Supersaturated
 Solvent holds more solute than is normally
possible at that temperature.
 These solutions are unstable; crystallization can
often be stimulated by adding a “seed crystal” or
scratching the side of the flask.
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Degree of saturation
Unsaturated, Saturated or Supersaturated?
 How much solute can be dissolved in a solution?
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Factors Affecting Solubility
• Chemists use the axiom
“like dissolves like”:
 Polar substances tend to
dissolve in polar solvents.
 Nonpolar substances tend
to dissolve in nonpolar
solvents.
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Factors Affecting Solubility
Example: ethanol in water
The stronger the
intermolecular
attractions between
solute and solvent,
the more likely the
solute will dissolve.
Ethanol = CH3CH2OH
Intermolecular forces = H-bonds; dipole-dipole; dispersion
Ions in water also have ion-dipole forces.
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Factors Affecting Solubility
Glucose (which has
hydrogen bonding) is
very soluble in water.
Cyclohexane (which
only has dispersion
forces) is not watersoluble.
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Factors Affecting Solubility
• Vitamin A is soluble in nonpolar compounds
(like fats).
• Vitamin C is soluble in water.
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Which
vitamin is
water-soluble
and which is
fat-soluble?
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Gases in Solution
• In general, the
solubility of gases in
water increases with
increasing mass.
Why?
• Larger molecules
have stronger
dispersion forces.
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Gases in Solution
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Gases in Solution
Increasing
pressure
above
solution
forces
more gas
to dissolve.
• The solubility of
liquids and solids
does not change
appreciably with
pressure.
• But, the solubility of a
gas in a liquid is
directly proportional to
its pressure.
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Henry’s Law
Sg = kPg
where
• Sg is the solubility of
the gas;
• k is the Henry’s law
constant for that gas in
that solvent;
• Pg is the partial
pressure of the gas
above the liquid.
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Temperature
Generally, the
solubility of solid
solutes in liquid
solvents increases
with increasing
temperature.
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Temperature
• The opposite is true of
gases. Higher
temperature drives
gases out of solution.
 Carbonated soft drinks
are more “bubbly” if
stored in the
refrigerator.
 Warm lakes have less
O2 dissolved in them
than cool lakes.
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