Transcript Fundamentals of General, Organic and Biological Chemistry 6th Edition Chapter Nine Solutions James E. Mayhugh Copyright © 2010 Pearson Education, Inc.

Fundamentals of General,
Organic and Biological
Chemistry
6th Edition
Chapter Nine
Solutions
James E. Mayhugh
Copyright © 2010 Pearson Education, Inc.
Outline
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9.1 Mixtures and Solutions
9.2 The Solution Process
9.3 Solid Hydrates
9.4 Solubility
9.5 The Effect of Temperature on Solubility
9.6 The Effect of Pressure on Solubility: Henry’s Law
9.7 Units of Concentration
9.8 Dilution
9.9 Ions in Solution: Electrolytes
9.10 Electrolytes in Body Fluids: Equivalents and Milliequivalents
9.11 Properties of Solutions
9.12 Osmosis and Osmotic Pressure
9.13 Dialysis
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Goals
►1. What are solutions, and what factors affect
solubility? Be able to define the different kinds of
mixtures and explain the influence on solubility of
solvent and solute structure, temperature, and pressure.
►2. How is the concentration of a solution
expressed? Be able to define, use, and convert between
the most common ways of expressing solution
concentrations.
►3. How are dilutions carried out? Be able to
calculate the concentration of a solution prepared by
dilution and explain how to make a desired dilution.
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Goals Contd.
►4. What is an electrolyte? Be able to recognize
strong and weak electrolytes and nonelectrolytes, and
express electrolyte concentrations.
►5. How do solutions differ from pure solvents in
their behavior? Be able to explain vapor pressure
lowering, boiling point elevation, and freezing point
depression for solutions.
►6. What is osmosis? Be able to describe osmosis
and some of its applications.
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9.1 Mixtures and Solutions
► Heterogeneous mixture: A nonuniform mixture that
has regions of different composition.
► Homogeneous mixture: A uniform mixture that has
the same composition throughout.
► Solution A homogeneous mixture that contains
particles the size of a typical ion or small molecule.
► Colloid A homogeneous mixture that contains
particles in the range 2–500 nm diameter.
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Liquid solutions, colloids, and heterogeneous
mixtures can be distinguished in several ways.
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9.2 The Solution Process
A good rule of thumb for predicting solubility is that
“like dissolves like”. Substances with similar
intermolecular forces form solutions and substances
with different intermolecular forces do not.
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Dissolution of an NaCl crystal in water. Polar water
molecules surround individual ions pulling them from
the crystal surface into solution. Oxygen atoms point
to (+) ions and hydrogen atoms point to (-) ions.
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9.3 Solid Hydrates
► Some ionic compounds attract water strongly enough to
hold onto water molecules even when crystalline,
forming what are called solid hydrates.
► Plaster of Paris, CaSO4·1/2H2O, is a solid hydrate. The
formula indicates that for every 2 formula units of
calcium sulfate in the crystal there is also one water.
► Still other ionic compounds attract water so strongly
that they pull water vapor from humid air to become
hydrated. Compounds that show this behavior, such as
calcium chloride are called hygroscopic and are often
used as drying agents.
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9.4 Solubility
► Hydrogen bonding between water and ethanol,
between water alone, and ethanol alone is so similar
that the two liquids are miscible, or mutually soluble
in all proportions.
► Most substances reach the limit of a saturated
solution: A solution that contains the maximum
amount of dissolved solute at equilibrium.
► Solubility: The maximum amount of a substance
that will dissolve in a given amount of solvent at a
specified temperature.
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9.5 The Effect of Temperature on
Solubility
► Temperature often has a dramatic effect on solubility.
► The effect of temperature is different for every
substance, however, and is usually unpredictable.
► Solids that are more soluble at high temperature than
at low temperature can sometimes form what are
called supersaturated solutions, which contain
even more solute than a saturated solution.
► Such a solution is unstable and precipitation can
occur dramatically when a tiny seed crystal is added.
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► Solubility of some (a)
solids and (b) gases, in
water as a function of
temperature.
► Most solid substances
become more soluble as
temperature rises.
► The solubility of gases
decreases as temperature
rises.
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9.6 The Effect of Pressure on
Solubility: Henry’s Law
Henry’s law: The solubility of a gas is directly
proportional to its partial pressure. If T is constant,
C  Pgas , or C/Pgas = k , or C1/P1 = C2/P2 .
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9.7 Units of Concentration
► Solute: A substance dissolved in a liquid.
► Solvent: The liquid in which a substance is dissolved.
► Solution: The combination of solute and solvent.
► A very useful means of expressing concentration in the
laboratory is molarity (M), the number of moles of
solute dissolved per liter of solution.
Moles of solute
Molarity (M) =
Liters of solution
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► Weight/Volume Percent Concentration [(w/v)%]
► Mathematically, (w/v)% concentration is found by
taking the number of grams of solute per milliliters of
solution and multiplying by 100.
► Volume/Volume Percent Concentration [(v/v)%]
► Mathematically (v/v)% is determined from the
volume of solute (usually in mL) per milliliter of
solution multiplied by 100.
(w/v)% concentration =
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Mass of solute (g)
Volume of solution (mL)
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x 100
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Parts per Million (ppm) and Parts per Billion
(ppb): When concentrations are very small, as often
occurs in dealing with trace amount of pollutants or
contaminants, parts per million (ppm) or parts per
billion (ppb) units are used.
Mass of solute (g)
ppm =
Mass of solution (g)
Mass of solute (g)
ppb =
Mass of solution (g)
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Volume of solute (mL)
x 106 or
x 106
Volume of solution (mL)
Volume of solute (mL)
x 109 or
x 109
Volume of solution (mL)
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► To prepare 100 mL of a specific solution, the solute
is measured out and dissolved in just enough solvent
to give a final volume of 100 mL.
► If the solute were dissolved in 100 mL of solvent, the
final volume of the solution will likely be a bit larger
or smaller than 100 mL.
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9.8 Dilution
► Dilution: Lowering concentration by adding
additional solvent.
► Dilution factor: The ratio of the initial and final
solution volumes (V1/V2).
► In the dilution process, the amount of solute
remains constant, only the volume is increased.
► Moles of solute = M1V1 = M2V2 = constant
► Dilution equations can be generalized to other
concentration units, C1V1 = C2V2
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► The following equation is very useful in calculating
final concentration of a solution after dilution.
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M2 V1 =M2V2
► M1 and V1 refers to the initial concentration and
volume of the solution and M2 and V2 refers to the
final concentration and volume of the solution.
► The final concentration will be equal to the product
of the initial concentration and the dilution factor.
►
M2 = M1 · (V1/V2).
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9.9 Ions in Solution: Electrolytes
► Electrolyte: A substance that produces ions and
therefore conducts electricity when dissolved in
water.
► Strong electrolyte: A substance that ionizes
completely when dissolved in water.
► Weak electrolyte: A substance that is only partly
ionized in water.
► Nonelectrolyte: A substance that does not produce
ions when dissolved in water.
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9.10 Electrolytes in Body Fluids:
Equivalents and Milliequivalents
► What happens if NaCl and KBr are dissolved in the
same solution? The cations and anions are all mixed
together so an identical solution could just as well be
made from KCl and NaBr. We can only speak of
having a solution with four different ions in it.
► A similar situation exists for blood and other body
fluids, which contain many different anions and
cations. Since they are all mixed together, it is
difficult to talk about specific ionic compounds.
► Instead, we are interested only in individual ions and
in the total numbers of positive and negative charges.
We need a new term, equivalents of ions.
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One equivalent (Eq) of an ion is an amount equal
to the molar mass of the ion divided by the number
of its charges:
Molar mass of ion (g)
One equivalent of ion =
Number of charges on ion
1 milliequivalent (mEq) = 0.001 equivalent (Eq)
1 Eq = 1000 mEq
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9.11 Properties of Solutions
► The properties of solutions are similar in many
respects to those of pure solvents, but there are
some important differences.
► The properties of a solution that depend on the
concentration of a dissolved solute but not on its
identity are known as colligative properties.
► Vapor pressures are lower, boiling points are higher
and freezing points are lower in solutions than they
are in a pure solvent.
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►For each mole of solute particles added the boiling
point of 1 kg of water is raised by 0.51°C. 1 mol of a
molecular substance like glucose raises the boiling
point of 1 kg of water from 100.0°C to 100.51°C. The
addition of 1 mol of NaCl per kilogram of water raises
the boiling point by 2 * 0.51°C = 1.02°C because the
solution contains 2 mol of solute particles.
►For each mole of solute particles, the freezing point of
1 kg of water is lowered by 1.86°C. 1 mol of antifreeze
per kilogram of water lowers the freezing point from
0.00°C to - 1.86°C and 1 mol of NaCl (2 mol of
particles) per kilogram lowers the freezing point from
0.00° C to -3.72°C.
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A close-up plot of vapor pressure versus temperature
for pure water (red curve) and for a 1.0 M NaCl
solution (green curve). Pure water boils at 100.0°C,
but the solution does not boil until 101.0°C.
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►A mixture of salt and ice is
used to provide the low
temperatures needed to
make old-fashioned handcranked ice cream.
►Daniel Fahrenheit did not
choose 32 degrees as the
freezing point of water, he
chose 0 degrees to be the
lowest temperature he
could reach in his lab by
mixing ice, water and salt.
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9.12 Osmosis and Osmotic Pressure
► Osmosis: The passage of solvent through a
semipermeable membrane separating two solutions
of different concentration.
► Osmotic pressure: The amount of external pressure
applied to the more concentrated solution to halt the
passage of solvent molecules across a
semipermeable membrane.
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► Osmolarity (osmol): The sum of the molarities of
all dissolved particles in a solution.
► Isotonic: Having the same osmolarity.
► Hypotonic: Having an osmolarity less than the
surrounding blood plasma or cells.
► Hypertonic Having an osmolarity greater than the
surrounding blood plasma or cells.
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► In (a) an isotonic solution, 0.30
osmol, the blood cells are normal in
appearance.
► The cells in (b) a hypotonic solution
are swollen because of water gain,
and may burst, a process called
hemolysis.
► Those in (c) a hypertonic solution
are shriveled because of water loss,
this process is called crenation.
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9.13 Dialysis
► Dialysis is similar to osmosis, except that the pores
in a dialysis membrane are larger than those in an
osmotic membrane so that both solvent molecules
and small solute particles can pass through, but large
colloidal particles such as proteins cannot pass.
► Hemodialysis is used to cleanse the blood of
patients whose kidneys malfunction. Blood is
diverted from the body and pumped through a long
cellophane dialysis tube suspended in an isotonic
solution formulated to contain many of the same
components as blood plasma.
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► Small waste materials such as urea pass through the
dialysis membrane from the blood to the solution
side where they are washed away.
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Chapter Summary
► Mixtures are classified as either heterogeneous or
homogeneous. Solutions are homogeneous mixtures
that contain particles the size of ions and molecules
whereas larger particles (2.0–500 nm diameter) are
present in colloids.
► The maximum amount of a solute that can be
dissolved in a solvent is called the solubility.
Substances tend to be mutually soluble when their
intermolecular forces are similar.
► The solubility in water of a solid often increases with
temperature, but the solubility of a gas decreases
with temperature.
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Chapter Summary Contd.
► Pressure significantly affects gas solubility, which is
directly proportional to the partial pressure over the
solution (Henry’s law).
► The concentration of a solution can be expressed in
several ways, including molarity, weight/weight
percent composition, weight/volume percent
composition, and parts per million.
► Molarity, the number of moles of solute per liter of
solution, is the most useful method when calculating
quantities of reactants or products for reactions in
aqueous solution.
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Chapter Summary Contd.
►A dilution is carried out by adding more solvent to an
existing solution. Only the amount of solvent
changes; the amount of solute remains the same.
►Substances that form ions when dissolved in water
and whose water solutions therefore conduct an
electric current are called electrolytes.
►Strong electrolytes ionize completely in water, weak
electrolytes ionize partially, and nonelectrolytes do
not ionize in water.
►Fluids containing many different electrolytes have
concentrations expressed in equivalents.
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Chapter Summary Contd.
►A solution has a lower vapor pressure, a higher boiling
point, and a lower melting point than a pure solvent.
►Colligative properties depend only on the number of
dissolved particles, not on their chemical identity.
►Osmosis occurs when solutions of different
concentration are separated by a semipermeable
membrane that allows solvent molecules to pass but
blocks the passage of solute ions and molecules.
►In dialysis, the membrane allows the passage of solvent
and small dissolved molecules but prevents passage of
proteins and larger particles.
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Key Words
►Colligative property
►Colloid
►Dilution factor
►Electrolyte
►Equivalent (Eq)
►Gram-equivalent
►Henry’s law
►Heterogeneous
mixture
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►Homogeneous mixture
►Hygroscopic
►Hypertonic
►Hypotonic
►Isotonic
►Miscible
►Molarity (M)
►Nonelectrolyte
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Key Words Contd.
►Osmolarity (osmol)
►Osmosis
►Osmotic pressure
►Parts per billion (ppb)
►Parts per million (ppm)
►Saturated solution
►Solubility
►Solute
►Solution
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►Solvation
►Solvent
►Strong electrolyte
►Supersaturated solution
►Volume/volume percent
concentration, (v/v)%
►Weak electrolyte
►Weight/volume percent
concentration, (w/v)%
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END OF CHAPTER 9
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