Chpt 13 Solutions
Solutions
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Solutions can be in many different forms.
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Solid – solid: solution of two solids, sterling silver
(Cu in Ag) or bronze (Zn in Cu).
Gas – gas: standard atomsphere is a solution of
multiple gases.
Solid – liquid: salt water
Solvent – the component of the solution
present in the greatest amount.
Solute – the component of the solution
present in the smaller amount.
Chpt 13: Solutions
CEM 152 – SS2012
Solution Process
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Consider the solution formed
from water and salt.
A salt crystal dropped in
water is surrounded by
water.
Polar water molecules
overcome strength of ionic
bonds.
Balance of forces between
solute and solvent and
between solute and solute or
solvent and solvent will
determine if a solution is
formed.
Chpt 13: Solutions
CEM 152 – SS2012
?
Solution process: Enthaply
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Conceptually three
different steps involved
in the solution process.
DHsoln = DHsolute +
DHsolvent + DHadd
Chpt 13: Solutions
CEM 152 – SS2012
Energetics
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The solution process is
A – always endothermic
B – always exothermic
C – neither endothermic nor exothermic
D – either endothermic or exothermic
Chpt 13: Solutions
CEM 152 – SS2012
Solution process:Enthaply
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Each step has an
associated
enthalpy change.
Overall process
can be either
exothermic or
endothermic.
Solute-solvent
interactions must
overcome solutesolute and
solvent-solvent
interactions.
Chpt 13: Solutions
separated solvent +
separated solute
separated solvent +
separated solute
DHsolvent
DHsolvent
Solvent +
separated solute
Solvent +
separated solute
DHsolute
DHadd
Solvent + solute
DHsoln
Solution
CEM 152 – SS2012
DHadd
DHsolute
Solution
DHsoln
Solvent + solute
Solution Process: Entropy
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Enthalpy change is only part of the solution
process.
In addition the amount of randomness (entropy)
must be considered. Process which increase
randomness proceed spontaneously.
This is the reason the NH4NO3 solution in water
proceeds spontaneously.
Entropy changes are typically significant because
the solute increases the volume in which it can
move.
In most cases solution formation is driven by an
increase in entropy.
Chpt 13: Solutions
CEM 152 – SS2012
Solution Process
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Don’t confuse the solution process with a true
chemical reaction.
A penny placed in HCl will disappear but the penny
has not truly formed a solution.
Instead a chemical reaction has occurred.
Zn (s) + 2 HCl (aq) -> ZnCl (aq) + H2 (g)
If NaCl is placed in water a solution is formed.
If the water is allowed to evaporate the NaCl is
recovered.
Chpt 13: Solutions
CEM 152 – SS2012
Solubility
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At the start of a solvation the solute
concentration begins to rise.
At some point the solute and solvent reaction an
equilibrium.
Solutions can be
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Saturated - no further solute will dissolve under these
conditions.
Undersaturated - more solute can be dissolved.
Supersaturated –excess of dissolved solute.
Solubility affected by three main components
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Intermolecular interactions
Pressure
Temperature
Chpt 13: Solutions
CEM 152 – SS2012
Factors affecting solubility:
Molecular Interactions
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Based solely on entropy you would
expect substances to be
completely soluble in each other.
Strong attraction between solute
Gas
and solvent result in higher
solubility.
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Due to dipole-dipole interactions polar N2
solutes will dissolve polar solvents.
O2
In water the ability to H-bond will
Ar
increase solubility.
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Miscible
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Immiscible
Chpt 13: Solutions
Kr
CEM 152 – SS2012
Solubility
(M)
A
0.69 x 10-3
B
1.38 x 10-3
C
1.50 x 10-3
D
2.79 x 10-3
Vitamin Solubility
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Your body’s need to ingest
vitamins on a daily basis
depends greatly on whether a
vitamin is fat soluble or water
soluble. Water soluble vitamins
are easily eliminated through
urine while fat soluble vitamins
are not. Which of the following
molecules are water soluble?
A – Vitamin C
B – Vitamins C, A
C – Vitamins C, B5
D – Vitamins A, B5
Vitamin B5
E – Vitamins A, K3
Chpt 13: Solutions
CEM 152 – SS2012
Vitamin C
Vitamin A
Vitamin K3
Factors affecting solubility:
Pressure
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Solubility of gases affected by pressure
Increasing the pressure of a gas above a
liquid increases the number of interactions
with the liquid surface.
A new equilibrium is established with a
higher solute concentration in the liquid.
k is Henry’s law constant and is different for
each solute-solvent pair.
Chpt 13: Solutions
CEM 152 – SS2012
Factors affecting solubility:
Temperature
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Solubility of most solid solutes in water increases as
the temperature increases.
Solubility of gases decreases with increasing
temperature.
Decrease in solubility is mainly due to increased
kinetic energy.
Decreased solubility at increased temperature is the
reason for thermal pollution of waterways
Also why a warm soda goes flat.
Chpt 13: Solutions
CEM 152 – SS2012
Solubility and Temperature
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A solution of 65 g of KNO3
in 100 g H2O at 50 oC is
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A – undersaturated
B – supersaturated
C – saturated
To form a saturated
solution of KClO3 in 100 g
H2O at 70 oC requires
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A – 10 g
B – 20 g
C – 30 g
D – 40 g
Chpt 13: Solutions
CEM 152 – SS2012
Boiling Water
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Question 1
Right before water begins
to boil small bubbles are
formed. What gas is the
main component in the
bubble?
A – N2
B – O2
C – H 2O
D – Ar
E – none of the above
Chpt 13: Solutions
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CEM 152 – SS2012
Question 2
When the water is boiling
larger bubbles are formed.
What gas is the main
component in the larger
bubbles?
A – N2
B – O2
C – H 2O
D – Ar
E – none of the above
Concentration
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Various ways of expressing
concentration.
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Mass percentage (ppm, ppb) – mass of
solute / mass solution
Mole fraction – moles of solute / total
moles
Molarity – moles solute / liters solution
Molality – moles solute / kg of solvent
Chpt 13: Solutions
CEM 152 – SS2012
Concentration
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Example
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Calculate the molality of 4.8 g of NaCl
dissolved in 0.350 L of water.
Chpt 13: Solutions
CEM 152 – SS2012
Concentration
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A commercial bleach solution is 3.62 mass
% NaOCl in water. Calculate molality,
molarity, and mole fraction.
Molality
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A–1m
B – 0.504 mol
C – 0.504 m
D – 0.486 m
E – 0.486 M
Chpt 13: Solutions
CEM 152 – SS2012
Colligative properties
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Colligative properties of solutions
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depend only on the amount of solute
and not its chemical identity
Four different effects
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Vapor pressure lowering
Freezing point depression
Boiling point elevation
Osmotic pressure
Chpt 13: Solutions
CEM 152 – SS2012
Lowering Vapor Pressure
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Vapor pressure of a solution is reduced compared
to a pure substance.
An ideal solution obeys Raoult’s Law
Raoult’s Law
The vapor pressure of pure water at 98 oC is 707
torr. Which solution has a higher vapor pressure,
10 g NaCl in 1000 kg of water or 10 g KI in 1000 kg
of water.
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A – NaCl, B – KI,
Chpt 13: Solutions
C - same
CEM 152 – SS2012
Lowering Vapor Pressure
For an ideal
solutions the vapor
pressure is lowered
in proportion to the
solute.
Po A
Total Pressure
Po B
Pressure
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partial
pressure
of A
partial
pressure
of B
0
0.2
0.4
0.6
Mole fraction of A
Chpt 13: Solutions
CEM 152 – SS2012
0.8
1
Distillation
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From Raoult’s Law alcohol distillation can begin.
Assume you have a liquid with two different
components, ethanol and water. How do you
separate out the alcohol?
Start with 2 mole water at 350C (vapor pressure
42.2 torr) and 1 mole of ethanol (vapor pressure
100 torr). What is the total pressure above the
solution?
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A – 28.1 torr
B – 61.4 torr
C – 33.3 torr
D – 14.1 torr
E – don’t know
Chpt 13: Solutions
CEM 152 – SS2012
Sealed beakers
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Assume you have two beakers of
water sealed within a larger
container. One beaker holds
pure water. The other holds a
hydrochloric acid solution. After
a long enough time what
happens to the water levels in the
two beakers? (Consider the
vapor pressures above both
solutions)
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A – 1 goes up, 2 goes down
B – 1 goes down, 2 goes up
C – 1 goes down, 2 goes down
D – 1 goes up, 2 goes up
E – nothing.
Chpt 13: Solutions
CEM 152 – SS2012
1
2
Vapor pressure lowering
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Lowering the
vapor pressure
of a solution will
affect the phase
diagram.
Chpt 13: Solutions
CEM 152 – SS2012
Boiling Point Elevation
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Lowering the vapor
pressure increases the
temperature at which the
vapor pressure is equal to
atmospheric pressure.
The boiling point elevation
is proportional to the
amount of solute
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DTb = Kbm
Note that molality is used in
the expression therefore
only the solvent maters.
Chpt 13: Solutions
CEM 152 – SS2012
solvent
water
benzene
ethanol
chloroform
b.p. (oC)
Kb (oC/m)
100
80.1
78.4
61.2
0.52
2.53
1.22
3.63
Freezing Point Depression
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The lowering of the vapor
pressure also depresses
the freezing point of a
solution below its normal
freezing point.
The freezing point
depression is proportional
to the amount of solute
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DTf = Kfm
Again, the depression is
only dependent on the
identity of the solvent.
Chpt 13: Solutions
CEM 152 – SS2012
solvent
water
benzene
ethanol
chloroform
f.p. (oC)
Kf (oC/m)
0.0
5.5
-115
-63
1.86
5.12
1.99
4.68
Freezing point depression and
boiling point elevation
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Antifreeze is a solution of ethylene glycol and water.
Calculate the changes in boiling point and freezing
point for a solution that is 45% ethylene glycol by
mass
Chpt 13: Solutions
CEM 152 – SS2012
Freeze-proof bug
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Certain bugs are able to withstand freezing
temperature by altering the solute concentration
within their body.
Assuming it is a water solution inside the bug what
molality is required to enable the bug to survive at
-5 oC. The molal freezing point depression constant
for H2O is 1.86 oC/m?
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A – 2.69 m
B – 1.86 m
C – 5.0 m
D – not enough information
Chpt 13: Solutions
CEM 152 – SS2012
Van’t Hoff factors
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Colligative properties will depend on the total
amount of solute independent of identity.
For example, calculate the boiling point of a 0.1 m
ethylene glycol solution and a 0.1 m NaCl solution
(Kb = 0.52 oC/m).
Tb EG = 100 + 0.1 * 0.51 * 1= 100.051
Tb NaCl = 100 + 0.1 * 0.51 * 2 = 100.102
The van’t Hoff factor describes the difference in
concentration due to the presence of a strong
electrolyte.
Chpt 13: Solutions
CEM 152 – SS2012
Van’t Hoff factors
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Which water solution will have the lowest freezing
point? Assume there is a total of 1 kg of H2O and
the volume does not change on addition of solute.
Molecular weights: Na - 23 g/mol, Cl – 35.4 g/mol,
Ca – 40 g/mol.
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Chpt 10: Gases
A – 7 g of C2H6O
B – 4 g of NaCl
C – 12 g of C6H8
D – 11 g of CaCl2
E – not enough information
CEM 152 – SS2012
46 g/mol
58.4 g/mol
80 g/mol
110.8 g/mol
Osmosis
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The movement of
solvent molecules
across a
semipermeable
membrane will
depend on the solute
concentration
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the net movement of
solvent is always
toward the solution
with the higher solute
concentration
Chpt 13: Solutions
CEM 152 – SS2012
Osmotic pressure
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The pressure required
to prevent osmosis is
known as the
osmotic pressure, p
The osmotic pressure
is found to obey a law
similar in form to the
ideal gas law
p = (n/V)RT = MRT
where M is the molarity of
the solution
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Isotonic
Chpt 13: Solutions
CEM 152 – SS2012
Molar Mass from osmosis
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A solution of a non-electrolyte is formed by
dissolving 2.35 g of a compound in water to form a
0.250 L solution. The osmotic pressure is 0.605
atm at 25oC. What is the molar mass?
Chpt 13: Solutions
CEM 152 – SS2012
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Why shouldn’t you drink salt water.
Chpt 13: Solutions
CEM 152 – SS2012
Question
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A helium balloon inflated one day will fall to the
ground the next day. The volume of the balloon
decreases somewhat overnight, but not enough to
explain why it no longer floats. If you inflate a new
balloon with helium to the same size as the balloon
that fell to the ground, the newly inflated balloon
floats. Explain why the helium balloon falls to the
ground overnight.
Chpt 13: Solutions
CEM 152 – SS2012
Colloids
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now
There is a range of
particle sizes that can
be dispersed in water.
If particles are larger
than molecules but
small enough a colloid
is formed.
Colloid particles range
between 0 – 1000 nm.
Chpt 13: Solutions
later
colloids
solutions
CEM 152 – SS2010
Colloids
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Individual colloid particles are large enough
to scatter light called the
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Tyndall effect
Colloids are dividing line between solutions
and heterogeneous mixtures.
Water is the most important dispersing
medium
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Hydrophilic
Hydrophobic
Chpt 13: Solutions
CEM 152 – SS2012
Colloids
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Hydrophobic particles can be stabilized by
adsorption of ions onto surface.
For example, sodium stearate
Stabilization occurs frequently in the body
Molecules form a liquid-liquid colloid called an
emulsion in small intestine to absorb fat soluble
molecules.
Colloidal particles can not be removed by filtration.
Colloidal particles must be removed by coagulation.
Chpt 13: Solutions
CEM 152 – SS2012