9.1 Mixtures and Solutions
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
9.12 Osmosis and Osmotic Pressure
9.13 Dialysis
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► 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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Homogeneous Mixtures
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Heterogeneous Mixtures
In a homogeneous mixture,
In a heterogeneous mixture,
• the composition of substances
is not uniform.
• the composition varies from
one part of the mixture to
another.
• the different parts of the
mixture are visible.
• the composition is uniform
throughout.
• the different parts of the
mixture are not visible.
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as Benjamin Cummings
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as Benjamin Cummings
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Liquid solutions, colloids, and heterogeneous
mixtures can be distinguished in several ways.
Classification of Matter
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9.2 The Solution Process
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.
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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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
9.5 The Effect of Temperature on
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.
► 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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9.6 The Effect of Pressure on
Solubility: Henry’s Law
► 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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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
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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.
Molarity (M) =
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Moles of solute
Liters of solution
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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► 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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► 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.
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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9.8 Dilution
► 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.
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M2 = M1 · (V1/V2).
► 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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9.9 Ions in Solution: Electrolytes
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► 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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9.12 Osmosis and Osmotic Pressure
One equivalent (Eq) of an ion is an amount equal
to the molar mass of the ion divided by the number
of its charges:
► 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.
Molar mass of ion (g)
One equivalent of ion =
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9.10 Electrolytes in Body Fluids:
Equivalents and Milliequivalents
► 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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Number of charges on ion
1 milliequivalent (mEq) = 0.001 equivalent (Eq)
1 Eq = 1000 mEq
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Osmosis
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• The 10% starch solution is diluted by the flow of
water out of the 4% and its volume increases.
• The 4% solution loses water and its volume
decreases.
• Eventually, the water flow between the two becomes
equal.
Suppose a semipermeable membrane separates a 4%
starch solution from a 10% starch solution. Starch is a
colloid and cannot pass through the membrane, but
water can. What happens?
semipermeable
membrane
starch
-10%
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Water Flow Equalizes
Osmosis
-4%
In osmosis,
• water (solvent) flows
from the lower solute
concentration into the
higher solute
concentration.
• the level of the solution
with the higher
concentration rises.
• the concentrations of the
two solutions become
equal with time.
-7%
starch
-H2O
-7%
starch
starch
-H2O
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Osmotic Pressure
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Learning Check
Osmotic pressure is
A semipermeable membrane separates a 10% sucrose
solution A from a 5% sucrose solution B. If sucrose is
a colloid, fill in the blanks in the statements below.
• produced by the solute particles dissolved in a
solution.
1. Solution ____ has the greater osmotic pressure.
• equal to the pressure that would prevent the flow of
additional water into the more concentrated
solution.
• greater as the number of dissolved particles in the
solution increases.
2. Water initially flows from ___ into ___.
3. The level of solution ____will be lower.
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Osmotic Pressure of the Blood
Solution
A semipermeable membrane separates a 10%
sucrose solution A from a 5% sucrose solution B. If
sucrose is a colloid, fill in the blanks in the
statements below.
Red blood cells
• have cell walls that are semipermeable membranes.
• maintain an osmotic pressure that cannot change or
damage occurs.
1. Solution A has the greater osmotic pressure.
2. Water initially flows from B into A.
3. The level of solution B will be lower.
• must maintain an equal flow of water between the
red blood cell and its surrounding environment.
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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.
► 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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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
Chapter Summary Contd.
► 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.
► 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.
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.
►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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