Properties of Solutions: Ways of Expressing Concentrations

1
Properties of Solutions Ways of Expressing
Concentrations
Weight percentage
Wt of component mass of component in solution
x 100
total mass of solution
Parts per million (ppm)
ppm of component mass of component in solution
x 106
total mass of solution
A solution is made containing 6.9 g NaHCO3
per 100 g water. What is the weight percentage
of solute in the solution? A 2.5 g sampe of
ground water was found to contain 5.4 micrograms
of Zn2. What is the concentration of Zn2 in ppm
6.9g
x 100 6.5
Wt of component
106.9 g
5.4 x 10-6
x 106
ppm
2.5 g
2
Properties of Solutions Ways of Expressing
Concentrations
Mole fraction, Molarity , and Molality
Mole fraction
Moles component
Mole fraction of a component
Total moles of all components
Calculate the mole fraction of HCl in a
solution of HCl containing 36 HCl by weight.
1 mole HCl
36 g HCl
0.99 mol HCl
36.5 g HCl
1 mole H2O
64 g H2O
3.6 mol H2O
18 g H2O
Mole HCl
0.99 mol

XHCl
0.22
Moles HCl H2O
4.6 mol
3
Properties of Solutions Ways of Expressing
Concentrations
Mole fraction, Molarity , and Molality
Moles of solute
Molarity
Liters of solution
What is the molarity of an ascorbic acid
solution (C6H8O6) prepared by dissolving 1.80
grams in enough water to make 125 mL of solution.
How many milliliters of this solution contain
0.0100 mol ascorbic acid.
1 mol C6H8O6
1.80 g C6H8O6
0.0102 mol C6H8O6
176 g C6H8O6
0.0102 mol C6H8O6
Molarity
0.0818 M
.125 L soln
4
Properties of Solutions Ways of Expressing
Concentrations
Mole fraction, Molarity , and Molality
Molality moles of solute
Kg of solvent
What is the molality of a solution made by
dissolving 5.0 g of toluene (C7H8) in 25 g of
benzene (C6H6)?
1 mol C7H8
5.0 g C7H8
0.054 mol C7H8
92 g C7H8
0.054 mol C7H8
Molality
2.2 m
0.025 kg C6H6
5
Properties of Solutions The Solution Process
Solvation or hydration
Cl-
-
-

-


-

6
Properties of Solutions The Solution Process
Energy Changes and Solution Formation
Solute-solute interactions
Solvent-solvent interactions
Solute-solvent interactions
? Hsoln ? H1 ? H2 ? H3
7
Properties of Solutions The Solution Process
Energy Changes and Solution Formation
NaOH ? Hsoln -44.48
NH4OH ? Hsoln 26.4
The overall change in enthalpy can be exo- or
endothermic
This explains why like dissolves like
8
Properties of Solutions Solution Formation,
Spontaneity and DisorderWHY DOES THIS STUFF
DISSOLVE?
London dispersion forces
bp 69
bp 77
Little energy is exchanged
9
Properties of Solutions Solution Formation,
Spontaneity and DisorderWHY DOES THIS STUFF
DISSOLVE?
Processes in which the energy content of the
system decreases tend to occur spontaneously.
Processes in which the disorder of the system
increases tend to occur spontaneously
When molecules of different types are brought
together, an increase in disorder occurs
spontaneously unless the molecules are
restrained sufficiently by strong intermolecular
forces. Salt does not dissolve in gasoline
because strong forces are holding the Na and Cl-
ions together and the intermolecular forces
associated with nonpolar substances is not
sufficient to dislodge them.
10
Properties of Solutions Saturated Solutions and
Solubility
As solid solute begins to dissolve in a solvent,
the concentrations of solute particles in
solution increases, so the chances of their
colliding with the surface of the
solid increases. This may lead to
crystallization.
Saturated a solution with undissolved solute
dissolve
Solute Solvent ? Solution
Supersaturated a solution which contains a
greater amount of solute than needed to form a
saturated solution
crystallize
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Properties of Solutions Factors Affecting
Solubility

• As a rule, solubility increases with increasing
  molecular mass

• Polar liquids tend to dissolve in polar solvents.
  If the dissolve they are miscible.
• If they do not dissolve they are immiscible

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Properties of Solutions Factors Affecting
Solubility
Hydrogen-boning interactions between solute and
solvent may lead to high solubility
Because of H-bonding, solute-solute,
solvent-solvent, and solute-solvent interactions
are not appreciably different.
Ethanol and water
Ethanol and ethanol
There is no significant change in the environment
13
Properties of Solutions Factors Affecting
Solubility
However, the numbers carbon atoms in an alcohol
does effect it solubility in water.
As the length of the chain increases, the OH
groups decrease leading to a decrease in
solubility
14
Properties of Solutions Factors Affecting
Solubility
Substances with similar intermolecular attractive
forces tend to be soluble in one another.LIKE
DISSOLVES LIKE
Glocuse has 5-OH groups on a six carbon framework
which makes the molecule fairly soluble in water
15
Properties of Solutions Factors Affecting
Solubility
Substances with similar intermolecular attractive
forces tend to be soluble in one another.LIKE
DISSOLVES LIKE
Predict whether each of the following substances
is more likely to dissolve in carbon tetrachloride
or water C7H16. NaHCO3, HCl
16
Properties of Solutions Pressure Effects
The solubility of a gas in any solvent is
increased as the pressure of the gas over the
solvent increases
Increased solubility of the gas can be explained
using LeChateliers Principle
By contrast the solubility of solids and liquids
are not appreciable affected by pressure.
17
Properties of Solutions Pressure Effects
The relationship between pressure and solubility
is expressed by Henrys Law
Cg kPg where Cg is the solubility of the gas in
the solution phase, Pg is the partial pressure of
the gas over the solution, and k is a Henrys law
constant which differs from one solute-solvent
pair to another.
For example the solubility of N2 gas in water at
25 C and 0.78 atm. is 5.3 x 10-4 M
k 5.3 x 10-4 M/0.78 6.8 x 10-4 mol/l-atm.
Assume that the partial pressure of the N2 is
doubled, Henrys law, Cg kPg predicts that the
solubility will also double.
Cg kPg
Cg (6.8 x 10-4 mol/l-atm)(1.56 atm)
Cg 0.0011
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Properties of Solutions Pressure Effects
The relationship between pressure and solubility
is expressed by Henrys Law
Calculate the concentration of CO2 in a soft
drink that is bottled with a partial pressure of
CO2 of 4.0 atm over the liquid at 25 C. The
Henry constant (k) for CO2 in water at this
temperature is 3.1 x 10-2 mol/l-atm
Cg kPg
Cg (3.1 x 10-2 mol/l-atm)(4.00 atm) 0.12
mol/L 0.12 M
For turn Calculate the concentration of CO2 in
a soft drink after the bottle is opened and sits
under a CO2 partial pressure of 3.0 x 10-4 atm
19
Properties of Solutions Temperature Effects
In general, the solubility of gases decreases as
temperature increases
20
Properties of Solutions Temperature Effects
In general, the solubility of of ionic compounds
normally increases as the temperature increases.
21
Properties of Solutions Colligative Properties
The increase or decrease of of the concentrations
and not the kinds of particles in solutions can
effect certain physical properties of solutions

• Lowering the freezing point of a solution

• Elevating the boiling point of a solution

• Reduction in vapor pressure

• Reduction in osmotic pressure

Colligative means depending upon the
collection colligative properties depend
upon the collective effect of the number of
solute particles
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Properties of Solutions Colligative Properties
Lowering the Vapor Pressure TRY EXPLAINING THIS
ONE!!
23
Properties of Solutions Colligative Properties
Lowering the Vapor Pressure Explaining How it
is Done!!
A non-volatile solute added to a solvent reduces
the capacity of the solvent molecules in the
liquid phase to move into the gas phase
The extent to which a nonvolatile solute lowers
the vapor pressure is proportional to the
concentration
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Properties of Solutions Colligative Properties
Raoults Law Calculating the vapor pressure of a
solution based upon the amount of solute being
added to the solution.
PA XAPA Where PA is the vapor pressure of the
solution, XA is the mole fraction of solvent, and
XAPA is the vapor pressure of the pure solvent
?P A PA - XAPA
?P A PA - XAP A
?P A PA (1 - XA)
Since XA XB 1 where XB is the mole fraction
of solute, XB 1 - XA
?P A XPA
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Properties of SolutionsBoiling Point
Elevation and Freezing Point Depression
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Properties of SolutionsBoiling Point
Elevation and Freezing Point Depression
Because nonvolatile solutes lower the vapor
pressure of a solution, a higher temperature is
required to cause the solution to boil.
Because the vapor pressure of the solution is
lower than that of the solvent at all temps, in
accordance to Raoults Law, the higher
temperature is required to attain a vapor
pressure of 1 atm.
? Tb Kb m , where ? Tb is directly
proportional to the number of solute particles
per moles of solvent molecules, and Kb is
called the molal boiling-point-elevation constant
For example, the Kb of water is 0.52 C/m
therefore a 1m solution of sucrose or any
other aqueous solution that is 1m in nonvolatile
solute particles will boil at a temperature of
0.52C higher than pure water.
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Properties of SolutionsBoiling Point
Elevation and Freezing Point Depression
The freezing point corresponds to the temperature
at which the vapor pressures of the solid and
liquid phases are the same. The freezing point of
a solution is lowered because the solute is not
normally soluble in the solid phase of the solvent
If the solute is nonvolatile, the vapor pressure
of the solution is reduced in proportion to the
mole fraction of solute.
? Tf Kf m , where ? Tf is directly
proportional to the number of solute particles
per moles of solvent molecules, and Kf is
called the molal-freezing-point-depression
constant
For example, the Kb of water is 1.86 C/m
therefore a 0.5 m solution of NaCl or any
other aqueous solution that is 1m in nonvolatile
solute particles will freeze at a temperature of
1.86 C lower than pure water.
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Properties of SolutionsBoiling Point
Elevation and Freezing Point Depression
Calculate the freezing point and the boiling
point of a solution of 100 g of ethylene glycol
(C2H6O2), antifreeze in 900 g of H2O.
Moles C2H6O2
100 g C2H6O2
1 mol C2H6O2
Molality

1.79 m
Kilograms H2O
.900 kg H2O
62.0 g C2H6O2
? Tf Kf m
1.86 C
(1.79 m )
? Tb Kbm
.052 C
3.33 C
(1.79 m )
m
m
Therefore freezing point -3.33 C
Therefore boiling point 100.93 C
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Properties of SolutionsOsmosis
The pressure required to prevent osmosis is
called the osmotic pressure, ?, of the solution,
where ? MRT
The net movement of solvent is always toward the
more concentrated solution
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Properties of SolutionsOsmosis
The pressure required to prevent osmosis is
called the osmotic pressure, ?, of the solution,
where ? MRT
31
Properties of SolutionsOsmosis
The pressure required to prevent osmosis is
called the osmotic pressure, ?, of the solution,
where ? MRT
Sample problem The average osmotic pressure of
blood is 7.7 atm. At 25C. What concentration of
glucose (C6H12O6) would be isotonic with blood?
32
Properties of SolutionsColloids
Suspensions that are the dividing Line Between
Solutions and Heterogeneous Mixtures
Tyndell effect a scattering of light by
colloidal particles
33
Properties of SolutionsColloids
The most important colloids are those in which
the dispersing medium is water
Hydrophilic and Hydrophobic Colloids
Hydrophilic colloids are kept in suspension by
interaction with surrounding water molecules
34
Properties of SolutionsColloids
The most important colloids are those in which
the dispersing medium is water
Hydrophilic and Hydrophobic Colloids
Hydrophobic colloids can be stabilized in water
by the adsorption of ions onto their surface
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Properties of SolutionsColloids
The most important colloids are those in which
the dispersing medium is water
Hydrophilic and Hydrophobic Colloids