CHAPTER FOURTEEN:
SOLUTIONS AND THEIR BEHAVIOR
SOLUTION CONCENTRATION
•Molarity(M):
Moles solute/1L solution
•Molality (m):
Moles solute/1kg solvent
•Mole fraction (XA):
•Mass percent:
Moles A*
total moles solution
Mass solute
total mass of solution
x 100
*In some applications, one needs the mole fraction of solvent, not solute.
Make sure you find the quantity you need!
CHANGING MOLARITY TO MOLALITY
If we know the
density of the
solution, we can
calculate the molality
from the molarity and
vice versa.
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CONVERTING MOLALITY TO MOLARITY
• Determine the molarity of a 0.273m aqueous
solution of KCl?
The density is 1.011g/mL
CONVERTING MOLALITY TO MOLARITY
• What is the molarity of a 0.273m aqueous
solution of KCl? The density if 1.011g/mL
• Ans: 0.271M
CONVERTING MOLARITY TO MOLALITY
• DETERMINE THE MOLALITY OF A 0.907M SOLUTION OF PB(NO3)2.
• THE SOLUTION DENSITY IS 1.252 G/ML.
CONVERTING MOLARITY TO MOLALITY
• An aqueous solution is 0.907M Pb(NO3)2. What is the
molaltiy of lead (II) nitrate in this solution? The density
is 1.252 g/mL.
• Ans: 0.953m
CALCULATING MASS PERCENT
• How would you prepare 425g of an aqueous solution
containing 2.40% by mass of sodium acetate?
• Ans: 10.2 g NaC2H3O2 in 414.8g of water
MOLE FRACTION
• Calculate the mole fraction of NaCl in an aqueous 5%
solution of NaCl
• Mole fraction NaCl = 0.016
SOLUTION CONCENTRATION
• Commercial concentrated aqueous ammonia is 27%
NH3 by mass and has a density of 0.90 g/mL. What is
the molarity of this solution?
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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METAL ALLOYS:
Substitutional alloys are formed when the two metallic
components have similar atomic radii and chemical-bonding
characteristics. For example, silver and gold form such an alloy
over the entire range of possible compositions.
METAL ALLOYS:
Interstitial alloys are form when the component present in the interstitial positions
between the solvent atoms must have a much smaller covalent radius than the solvent
atoms. Typically, an interstitial element is a nonmetal that participates in bonding to
neighboring atoms.
The presence of the extra bonds provided by the interstitial component causes the metal
lattice to become harder, stronger, and less ductile. For example, steel is an alloy of iron
that contains up to 3 percent carbon. Steel is much harder and stronger than pure iron.
AQUEOUS SOLUTIONS
The intermolecular forces
between solute and
solvent particles must be
strong enough to
compete with those
between solute particles
and those between
solvent particles.
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HOW DOES A SOLUTION FORM?
As a solution forms, the solvent pulls solute
particles apart and surrounds, or solvates,
them.
http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/molvie1.swf
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HOW DOES A SOLUTION FORM
If an ionic salt is
soluble in water, it is
because the iondipole interactions
are strong enough to
overcome the lattice
energy of the salt
crystal.
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The Solution Process
• Solutes dissolve in solvents by a process called
solvation.
• Polar solvent dissolve polar solutes, non-polar
solvents dissolve non-polar solutes. (aka: “like
dissolves like”.
• If two liquids mix to an appreciable extent to form a
solution, they are said to be miscible.
• In contrast, immiscible liquids do not mix to form a
solution; they exist in contact with each other as
separate layers.
Solvation of Ions
+
When a cation exists in solution, it is surrounded by the negative
dipole ends of water molecules.
When as anion exists in solution, it is surrounded by the positive
dipole ends of water molecules.
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
The more similar
the intermolecular
attractions, the
more likely one
substance is to
be soluble in
another.
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ENERGY CHANGES IN SOLUTION
THE enthalpy change
of the overall process
depends on H for
each of these steps.
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WHY DO ENDOTHERMIC PROCESSES
OCCUR?
Things do not tend to occur
spontaneously (i.e., without
outside intervention) unless
the energy of the system is
lowered.
The reason is that increasing
the disorder or randomness
(known as entropy) of a
system tends to lower the
energy of the system.
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TYPES OF SOLUTIONS
• Saturated
• In a saturated solution,
the solvent holds as
much solute as is
possible at that
temperature.
• Dissolved solute is in
dynamic equilibrium with
solid solute particles.
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TYPES OF SOLUTIONS
• Unsaturated
• If a solution is
unsaturated, less
solute than can
dissolve in the
solvent at that
temperature is
dissolved in the
solvent.
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TYPES OF SOLUTIONS
• Supersaturated
• In supersaturated solutions, the solvent holds more
solute than is normally possible at that
temperature.
• These solutions are unstable; crystallization can
usually be stimulated by adding a “seed crystal” or
scratching the side of the flask.
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GASES IN SOLUTION
• In general, the
solubility of gases in
water increases with
increasing mass.
• Larger molecules have
stronger dispersion
forces.
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GASES IN SOLUTION
• The solubility of liquids
and solids does not
change appreciably
with pressure.
• 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, and
• 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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SUCROSE SOLUBILITY
TEMPERATURE
• The opposite is true of
gases.
• 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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COLLIGATIVE PROPERTIES
• In chemistry, colligative properties are properties of
solutions that depend upon the ratio of the number of
solute particles to the number of solvent molecules in
a solution, and not on the type of chemical species
present.
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COLLIGATIVE PROPERTIES:
•Vapor Pressure
•Freezing Point Depression
•Boiling Point Elevation
•Osmotic Pressure
VAPOR PRESSURE
Because of solutesolvent intermolecular
attraction, higher
concentrations of
nonvolatile solutes
make it harder for
solvent to escape to
the vapor phase.
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RAOULT’S LAW
PA = XAPA
where
• XA is the mole fraction of compound A, and
• PA is the normal vapor pressure of A at that
temperature.
NOTE: This is one of those times when you want to
make sure you have the vapor pressure of the 2009,©
Prenti
cesolvent.
Hall,
Inc.
VAPOR PRESSURE:
• Raoult’s Law: Psolution = XsolventPoH2O
12g Sucrose (C12H22O11)is dissolved in 250.0g water at
90 °C.
What is the vapor pressure of water over this solution?
(PoH2O = 525.8 mmHg- from data table)
• Ans. 524 mmHg
BOILING POINT ELEVATION AND
FREEZING POINT DEPRESSION
Nonvolatile solutesolvent interactions
also cause solutions to
have higher boiling
points and lower
freezing points than
the pure solvent.
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BOILING POINT ELEVATION
• The change in boiling point
is proportional to the
molality of the solution:
Tb = Kb  m
Tb is added to the normal boiling
point of the solvent.
where Kb is the molal
boiling point elevation
constant, a property of the
solvent.
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BOILING POINT ELEVATION AND
FREEZING POINT DEPRESSION
Note that in both
equations, T does not
depend on what the solute
is, but only on how many
particles are dissolved.
TB = KB  m
TF = KF  m
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2009,
Prenti
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Inc.
FREEZING POINT DEPRESSION:
• Freezing Point Depression: ΔTfp = Kfpmsolution
• A solution is prepared by adding 0.50g of caffeine (C8H10O2N4) to
100g of benzene (C6H6).
• Calculate the freezing point of this solution.
• The freezing point of pure benzene is 5.50°C
• Kfp for benzene = 5.23 °C/m)
ANS: TF = 0.132 ° C
TF = 5.37C
BOILING POINT ELEVATION:
• Boiling point elevation: ΔTbp = Kbpmsolute
• A glycerol solution (C3H8O3) in water is prepared by
dissolving glycerol is 500g water. The boiling point of the
solution is 100.42°C at 760mmHg. What mass of glycerol
was dissolved to make this solution?
• Kbp = 0.5121 °C/m
Ans: 38 g glycerol
COLLIGATIVE PROPERTIES OF
ELECTROLYTES
Since these properties depend on the number of
particles dissolved, solutions of electrolytes (which
dissociate in solution) should show greater changes
than those of nonelectrolytes.
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VAN’T HOFF FACTOR
• We modify the
previous equations
by multiplying by the
van’t Hoff factor, i.
Tf = Kf  m  i
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VAN’T HOFF FACTOR
• Re-association is more
likely at higher
concentration.
• Therefore, the number
of particles present is
concentrationdependent.
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ROLE OF ELECTROLYTES ON
COLLIGATIVE PROPERTIES:
• Van’t Hoff Factor: ΔTfp = Kfpmsolutei
• A 0.0711m aqueous solution of Sodium sulfate freezes at
-0.32C. What is the actual value (i) of the van’t Hoff
factor? Kfp = 1.86 °C/m
Ans: i = 2.42
RANK FOR FOLLOWING:
• Increasing boiling point (ΔTb)
• 0.25 m C6H12O11
• 0.40 m NaCl
• 0.15 m MgCl2
C6H12O11  MgCl2  NaCl
OSMOSIS
In osmosis, there is net movement of solvent from
the area of higher solvent concentration (lower
solute concentration) to the are of lower solvent
concentration (higher solute concentration).
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OSMOTIC PRESSURE
The pressure required to stop osmosis, known as
osmotic pressure, ()
=(
n
)
RT = MRT
V
where M is the molarity of the solution.
If the osmotic pressure is the same on both sides of a membrane
(i.e., the concentrations are the same), the solutions are isotonic.
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2009,
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OSMOSIS IN BLOOD CELLS
• If the solute
concentration outside the
cell is greater than that
inside the cell, the
solution is hypertonic.
• Water will flow out of the
cell, and crenation
results.
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OSMOTIC PRESSURE
• Osmotic Pressure: π= MRT
• What is the osmotic pressure of a 0.1M solution of
sucrose at 25C?
(remember: R= .08206 L-atm/mol-K)
Ans: 2.45 atm
ANOTHER OSMOTIC PRESSURE
PROBLEM.
• 100. mg of a protein are dissolved in enough water to
make 10.0 mL of a solution.
• If this solution has an osmotic pressure of 13.3
mmHg at 25°C, what is the molar mass of the
protein?
Ans: 1.4 x 104 g/mole
COLLOIDS
Suspensions of particles larger than individual ions
or molecules, but too small to be settled out by
gravity are called colloids.
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TYNDALL EFFECT
• Colloidal suspensions can scatter
rays of light.
• This phenomenon is known as the
Tyndall effect.
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How Soap Works:
Soap molecules interact with water through the charged,
hydrophilic end of the molecule.
The long, hydrophobic end of the molecule binds through
dispersion forces with nonpolar hydrocarbons and other non
polar substances.