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• Electrolytes (Conductive):
Dissociation in to ions in
• Nonelectrolytes (no conductivity):
Concentration expressions for solutions
Percent by weight % w/w
Percent by volume %v/v
Percent weight in volume % w/v
Concentration expressed as percentage
– Percent weight-in-weight (w/w) is the grams of solute in
100 grams of the solution.
– Percent weight-in-volume (w/v) is the grams of solute in
100ml of the solution.
– Percent volume-in-volume (v/v) is the milliliters of solute
in 100ml of the solution.
A 20 % w/w solution contains 20g solute how many grams the solvent is?
Molarity, Normality, and Molality
Molarity and normality both depend on the volume of
the solvent, so their values are affected by change of
volume caused by factors such as change in
Molality doesn’t has this disadvantage.
In a solution containing 0.01 mole of solute and 0.04
mole of solvent, the mole fraction of the solute is 0.2
and for solvent it is 0.8
Mole percent = mole fraction X 100
An aqueous solution of ferrous sulfate was prepared
by adding 41.50 g of FeSO4 to enough water to
make 1000 mL of solution. The density of the
solution is 1.0375 and the molecular weight of
FeSO4 is 151.9.
(c) mole fraction of FeSO4, mole fraction of water,
and the mole percent of the two constituents
(d) % w/w of FeSO4.
Ideal and real solutions
Ideal solution is defined as a solution in which there is
no change in the properties of the components other
than dilution when they are mixed to form the solution
Molecules exhibit complete freedom of motion and
randomness of distribution in the solution.
Ideality in solutions means complete uniformity of
Ideal Solutions and
P = pA + pB
pA = pA◦ XA
pB = pB◦ XB
What is the partial vapor pressure of benzene and of
ethylene chloride in a solution at a mole fraction of
benzene of 0.6? The vapor pressure of pure benzene at
50◦C is 268 mm, and the corresponding pA ◦ for
ethylene chloride is 236 mm.
Raoult's Law States that, in an ideal solution, the partial vapor pressure of each volatile
constituent is equal to the vapor pressure of the pure constituent multiplied by its
mole fraction in the solution. Thus, for two constituents A and B,
Ideality in solutions presupposes complete uniformity
of attractive forces.
Many examples of solution pairs are known,
however, in which the “cohesive” attraction of A for A
exceeds the “adhesive” attraction existing between A
Similarly, the attractive forces between A and B may
be greater than those between A and A or B and B.
Such mixtures are real or nonideal; that is, they do
not adhere to Raoult’s law
Two types of deviation from Raoult’s law are
recognized, negative deviation and positive
When the “adhesive”
molecules of different
species exceed the
between like molecules,
the vapor pressure of
the solution is less than
that expected from
Raoult’s ideal solution
law, and negative
When the “adhesive”
molecules of different
species are weaker
attractions between like
molecules, the vapor
pressure of the solution
is more than that
expected from Raoult’s
ideal solution law, and
Colligative properties of solutions are those that
affected (changed) by the presence of solute and
depend solely on the number (amount of solute in
the solutions) rather than nature of constituents.
Examples of colligative properties are:
Colligative vs Non-colligative
Compare 1.0 M aqueous sugar solution to a 0.5 M solution of
salt (NaCl) in water.
both solutions have the same number of dissolved particles
any difference in the properties of those two solutions is due to
a non-colligative property.
Both have the same freezing point, boiling point, vapor
pressure, and osmotic pressure
Sugar solution is sweet and salt solution is salty.
Therefore, the taste of the solution is not a colligative
Another non-colligative property is the color of a solution.
Other non-colligative properties include viscosity, surface
tension, and solubility.
Lowering of vapor pressure
Pure solvent > solutions
Lowering of vapor pressure
According to raoult’s law Psolvent = Pºsolvent Xsolvent
But if the solute used in non volatile only pressure from
solvent can be considered.
On the other hand
Psolute = Pºsolute Xsolute
Psolution = Pºsolvent Xsolvent
X1 = mole fraction of solvent
X2 = mole fraction of solute
∆p = p1◦ − p is the lowering of the vapor pressure
and ∆p/p1◦ is the relative vapor pressure lowering.
The relative vapor pressure lowering depends only
on the mole fraction of the solute, X2, that is, on the
number of solute particles in a definite volume of
solution. Therefore, the relative vapor pressure
lowering is a colligative property.
Calculate the relative vapor pressure lowering at 20◦C
for a solution containing 171.2 g of sucrose (w2) in 100
g (w1) of water. The molecular weight of sucrose (M2)
is 342.3 and the molecular weight of water (M1) is
Boiling point elevation
Boiling point elevation is a colligative property related to
vapor pressure lowering.
The boiling point is defined as the temperature at which
the vapor pressure of a liquid equals the atmospheric
Due to vapor pressure lowering, a solution will require a
higher temperature to reach its boiling point than the pure
Elevation of the Boiling Point
The boiling point of a solution of a nonvolatile solute
is higher than that of the pure solvent owing to the
fact that the solute lowers the vapor pressure of the
ΔTb = K X2
ΔTb = Kbm
boiling point is a colligative property
In dilute solutions:
ΔTb = K X2 ΔTb = Kbm
Tb: is known as the boiling point elevation
Kb: is called the molal elevation constant.
m: is molality of solvent
Every liquid has a freezing point - the temperature at
which a liquid undergoes a phase change from liquid to
When solutes are added to a liquid, forming a solution, the
solute molecules disrupt the formation of crystals of the
That disruption in the freezing process results in a
depression of the freezing point for the solution relative to
the pure solvent.
Depression of the Freezing Point
∆T f = Tº f – T f
Kf is the molal epression constant
When a solution is separated
from a volume of pure solvent
by a semi-permeable
membrane that allows only the
passage of solvent molecules,
the height of the solution
begins to rise.
The value of the height
difference between the two
compartments reflects a
property called the osmotic
pressure of a solution.
π is the osmotic pressure .
V is the volume of the solution in liters.
n is the number of moles of solute.
R is the gas constant, equal to 0.082 liter atm/mole deg.
T is the absolute temperature.
Van't Hoff and Morse Equations for Osmotic Pressure:
MOLECULAR WEIGHT DETERMINATION
The four colligative properties can be used to calculate
the molecular weights of nonelectrolytes present as
solutes. Using vapor pressure lowering
Using boining point elevation
Using Freezing point depression
Using Osmotic pressure