Properties of Solutions
Classification
of Matter
Solutions are
homogeneous
mixtures
Solute
A solute is the dissolved substance in a
solution.
Salt in salt water
Sugar in soda drinks
Carbon dioxide in soda drinks
Solvent
A solvent is the dissolving medium in a
solution.
Water in salt water
Water in soda
Calculations of Solution Concentration
Mass percent - the ratio of mass (in
grams) of solute to mass (in grams) of
solution, expressed as a percent
 mass of solute ( g ) 
 x 100
Mass percent  
 mass of solution ( g ) 
Calculations of Solution Concentration
Mass/volume (m/v) % - the ratio of mass
(in grams) of solute to volume of solution
(in mL), expressed as a percent
 mass of solute( g ) 
 x 100
%( m / v)  
 volume of solution (mL) 
Calculations of Solution Concentration
Volume/volume (v/v) % - the ratio of
volume (in mL) of solute to volume of
solution (in mL), expressed as a percent
 volume of solute(mL ) 
%(v / v)  
 x 100
 volume of solution(mL ) 
Calculations of Solution Concentration
Mole fraction – the ratio of moles of
solute to total moles of solution
nA
Mole fraction of A   A 
n A  nB
Calculations of Solution Concentration
Molarity (M) - the ratio of moles of solute
to liters of solution
moles of solute
Molarity  M 
Liter of solution
Calculations of Solution Concentration
Normality (N) – moles of equilvalents/Liter
of solution
moles _ of _ equivalents
Normality  N 
Liter _ of _ solution
Calculations of Solution Concentration
Molality (m) – moles of solute per
kilogram of solvent
moles solute
Molality  m 
kilo gram solvent
Dissolving Stuff
“Like Dissolves Like”
Nonpolar solutes dissolve best in nonpolar solvents
Fats
Benzene
Steroids
Hexane
Waxes
Toluene
Polar and ionic solutes dissolve best in polar solvents
Inorganic Salts
Sugars
Water
Small alcohols
Acetic acid
Heat of Solution
The Heat of Solution is the amount of heat
energy absorbed (endothermic) or released
(exothermic) when a specific amount of
solute dissolves in a solvent.
Substance
Heat of Solution
(kJ/mol)
NaOH
-44.51
NH4NO3
+25.69
KNO3
+34.89
HCl
-74.84
Steps in Solution Formation
H1 Expanding the solute
Separating the solute into individual
components
H2 Expanding the solvent
Overcoming intermolecular forces of
the solvent molecules
H3 Interaction of solute and solvent to
form the solution
Enthalpy Changes in Solution
The enthalpy
change of the
overall process
depends on H for
each of these steps.
Start
End
Start
End
Why do endothermic processes
sometimes occur spontaneously?
Some processes,
like the dissolution
of NH4NO3 in water,
are spontaneous at
room temperature
even though heat is
absorbed, not
released.
Enthalpy Is Only Part of the Picture
Entropy is a measure of:
• Dispersal of energy in
the system.
• Number of microstates
(arrangements) in the
system.
b. has greater entropy,
 is the favored state
(more on this in chap 19)
Predicting Solution Formation
Solvent/
Solute
Polar/
Polar
Polar/
Nonpolar
Nonpolar/
Nonpolar
Nonpolar/
polar
H1
H2
H3
+ large + large - large
+ small + large
+/small
+ small + small
+/small
+ large + small
+/small
Hsol’n
+/small
Outcome
Solution
forms
+ large No solution
forms
+/Solution
small
forms
+ large No solution
forms
Colligative Properties
• Colligative properties depend only on
the number of solute particles
present, not on the identity of the
solute particles.
• Among colligative properties are
Vapor pressure lowering
Boiling point elevation
Freezing point depression
Osmotic pressure increase
Vapor Pressure
As solute molecules are
added to a solution, the
solvent becomes less
volatile (=decreased
vapor pressure).
Solute-solvent interactions
contribute to this effect.
Raoult’s Law
The presence of a nonvolatile solute lowers
the vapor pressure of the solvent.
Psolution  
0
solvent solvent
P
Psolution = Observed Vapor pressure of
the solution
solvent = Mole fraction of the solvent
P0solvent = Vapor pressure of the pure solvent
What is the vapor pressure in mmHg of
a solution that contains 155 grams of
glucose dissolved in 250. mL of water
at 25.00 C? The vapor pressure of pure
water at 25.00C is 23.78 mmHg.
22.4 mmHg
Liquid-liquid solutions in which both
components are volatile
Modified Raoult's Law:
PTOTAL  PA  PB   A P   B P
0
A
0
B
P0 is the vapor pressure of the pure solvent
PA and PB are the partial pressures
At a given temperature, you have a mixture of
benzene (vapor pressure of pure benzene = 745
torr) and toluene (vapor pressure of pure
toluene = 290. torr). The mole fraction of
benzene in the solution is 0.450. Assuming
both substances are volatile, calculate the
vapor pressure of the solution.
495 torr
Colligative Properties of
Electrolytes
Because these properties depend on the number of
particles dissolved, solutions of electrolytes (which
dissociate in solution) show greater changes than
those of nonelectrolytes.
e.g. NaCl dissociates to form 2 ion particles; its limiting
van’t Hoff factor is 2.
The van’t Hoff Factor,
i
Electrolytes may have two, three or more
times the effect on boiling point, freezing
point, and osmotic pressure, depending on
its dissociation.
Dissociation Equations and the
Determination of i
i = 2
NaCl(s)  Na+(aq) + Cl-(aq)
AgNO3(s)  Ag+(aq) + NO3-(aq)
i = 2
i = 3
MgCl2(s)  Mg2+(aq) + 2 Cl-(aq)
Na2SO4(s)  2 Na+(aq) + SO42-(aq)
i = 3
AlCl3(s)  Al3+(aq) + 3 Cl-(aq)
i = 4
van’t Hoff Factor
One mole of NaCl in
water does not
really give rise to
two moles of ions.
van’t Hoff Factor
Some Na+ and Cl−
reassociate as
hydrated ion pairs,
so the true
concentration of
particles is
somewhat less than
two times the
concentration of
NaCl.
The van’t Hoff Factor
• Reassociation is more likely at higher
concentration.
• Therefore, the number of particles present
is concentration dependent.
Boiling Point Elevation and
Freezing Point Depression
Solute-solvent
interactions also
cause solutions to
have higher boiling
points and lower
freezing points than
the pure solvent.
Boiling Point Elevation and
Freezing Point Depression
In both equations,
T does not depend
on what the solute
is, but only on how
many particles are
dissolved.
Tb = Kb  i m
Tf = Kf  i m
Boiling Point Elevation
Each mole of solute particles raises the
boiling point of 1 kilogram of water by
0.51 degrees Celsius.
T  i  K b  msolute
Kb = 0.51 C  kilogram/mol
m = molality of the solution
i = van’t Hoff factor
Boiling Point Elevation
The change in boiling point is proportional to
the molality of the solution:
Tb = Kb i m
where Kb is the molal boiling point elevation
constant, a property of the solvent.
Tb is added to the normal
boiling point of the solvent.
If I add 45.0 grams of sodium chloride
to 500. grams of water, what will the
boiling point be of the resulting
solution? Kb(H2O) = 0.512 0C/m
101.57 ˚C
Freezing Point Depression
Each mole of solute particles lowers the
freezing point of 1 kilogram of water by
1.86 degrees Celsius.
T  i  K f  m solute
Kf = 1.86 C  kilogram/mol
m = molality of the solution
i = van’t Hoff factor
Freezing Point Depression
• The change in freezing point can be found
similarly:
Tf = Kf i m
• Here Kf is the molal freezing point depression
constant of the solvent.
Tf is subtracted from the normal
freezing point of the solvent.
If I add 92.0 grams of sodium
chloride to 500. grams of water,
what will the freezing point be of
the resulting solution? K (H2O) =
1.86 0C/m
-11.7 ˚ C
Solvent
Kf
Kb
Acetic acid
3.90
3.07
Benzene
5.12
2.53
8.1
5.24
Phenol
7.27
3.56
Water
1.86
0.512
Nitrobenzene
52
59
#tryproblem79
Osmotic Pressure
Osmotic Pressure
• The pressure required to stop
osmosis, known as osmotic pressure,
, is
n
=i
RT = MiRT
(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.
What is the osmotic pressure (at
85.0°C) of saltwater that contains
approximately 27.0 grams of NaCl
per liter with an observed van’t
Hoff factor of 1.89.
25.7 atm
i
i
Molar Mass from
Colligative Properties
125 mg of an Alkaline Earth metal
chloride (XCl2) dissolved in enough
water to make 50.0 mL of a solution
at 25 ˚C that has an osmotic
pressure of 881.6 mmHg. Identify
the Alkaline Earth Metal!!!!