Colligative properties of
nonelectrolyte’s solutions
PLAN
1. The main concepts of solutions
2. Types of solutions
3. Heat effect of a dissolution
4. Methods for expressing the concentration of a solution
5. Vapour pressure and Raoult’s law
6. Collogative properties:
• 6.1 Relative lowering in vapour pressure
• 6.2 Elevation in boiling point
• 6.3 Depression in freezing point
• 6.4 Osmotic pressure
7 Van’t Hoff factor ‘i’
Assistant Kozachok S.S. prepared
A solution is a homogeneous
mixture of two or more substances
whose composition can be varied
within certain limits
The substances making up the
solutions are called components
• The components of a binary solution are solute
and solvent.
• Solvent is a component which is present in
excess, in other words a solvent is a substance
in which dissolution takes place. Solvent doesn’t
change its physical state during reaction of
dissolution.
• Solute is a component which is present in lesser
quantity. Or solute is a substance that dissolves
In a solution, the particles are of molecular size (about
1000 pm) and the different components cannot be separated by
any of the physical methods such as filtration, setting
,centrifugation, etc.)
TYPES OF SOLUTION
1. Depending upon the total components
present in the solution:
a) Binary solution (two components)
b) Ternary solution (three components)
c) Quaternary solution (four components)…..etc.
2. Depending upon the ability of the dissolution
some quantity of the solute in the solvent:
a) Saturated solution
b) Not saturated solution
3. Depending upon the physical states of the solute and solvent, the
solution can be classified into the following nine type:
Out of the nine types of solutions, namely solid in liquid, liquid in liquid and
gas in liquid are very common. In all these types of solutions, liquid acts as
solvent.
4. According to the nature of solvent the solutions can be classified
such as: a) aqueous solution – the solution in which water is a solvent;
b) non- aqueous solution in which water is not the solvent (ether, benzene…)
The basic rule for solubility is “like dissolves like”
5. Depending upon component’s solubility in liquid solutions (which are
themselves liquids), these mixtures may be classified into the following three types:
1) The two components are completely miscible (ethyl alcohol in water)
2) The two components are almost immiscible (oil and water, benzene and water)
3) The two components are partially miscible (ether and water)
6. The binary solutions may be classified into two types:
1) Ideal solutions. Such solutions are formed by mixing the two components
which are identical in molecular size, in structure and have almost identical
intermolecular forces. In these solutions, the intermolecular interactions between
the components (A-B) are of same magnitude as the intermolecular interactions in
pure components ( A-A and B-B). Ideal solutions obeys Raoult’s law.
2) Non-ideal solutions
The mechanism of dissolving
NaCl in the water
Cl-
Na+
Cl-
Methods for expressing the
concentration of a solution
The concentration of a solution may be defined as the amount of
solute present
In the given quantity of the solution.
1. Mass percentage or volume percentage
The mass percentage of a component in a given solution is the mass
of the com ponent per 100 g of the solution.
2. Molarity
It is the number of moles of the solute dissolved per litre of
the solution. It’s represented as M or
(М)
= Moles of solute / Volume of solution in litres
or
(М)
= Mass of component A/ Molar mass of A *Volume
of solution in litres
The unit of molarity is mol/L, 1L = 1000 ml
3. Molality
It is the number of moles of the solute dissolved per 1000 g (or 1
kg) of the solvent. It’s denoted by m or
(m)
= Moles of solute/Weight of solvent in kg
or
(m)
= Moles of solute * 1000/Weight of solvent in gram
The unit of Molality is m or mol/kg
Molalty is considered better for expressing the concentration
as compared to molarity because the molarity changes with
temperature because of expansion of the liquid with the
temperature
4. Normality
It is the number of gram equivalents of the solute dissolved per
litre of the solution. It’s denoted by N or
(N) = Number of gram equivalents of solute/Volume of
solution in litres
or
(N)
= Number of gram equivalents of solute *1000/Volume
of solution in ml
Number of gram equivalents of solute = Mass of solute /
Equivalent mass of solute
Methods f expression of
solution’s composition
Mass
Volume
Mass particle ω,
Molarity См,
[%]
[mol/L]
Molar particle χ,
Normality СN,
[%]
[mol-equivalent./L]
Molality Сm,
Titre Т,
[mol/kilogram]
[g/ml]
Relationship between Normality and Molarity of Solutions
Normality = Molarity * Molar mass/Equivalent mass
5. Mole fraction
It is the ratio of number of moles of one component to the
total number of moles (solute and solven) present in the
solution. It’s denoted by X. Let suppose that solution contains
moles of solute and
moles of the solvent. Then
Vapour pressure and Raoult’s law
The pressure exerted by the vapours above the liqud
surface in equilibrium with the liquid at a given
temperature is called vapour pressure
The vapour pressure of a liquid depends upon
1. Nature of the liquid. The liquid, which have weaker
intermolecular forces, tend to escape readily into vapour
phase and therefore, have greater vapour pressure.
2. Temperature. The vapour pressure of a liquid increases
with increase in temperature. This is due to the fact that
with increase in temperature, more molecules will have
large kinetic energies. Therefore, larger number of
molecules will escape from the surface of the liquid to the
vapour phase resulting higher vapour pressure.
The process of evaporation in a closed container will proceed until
there are as many molecules returning to the liquid as there are
escaping. At this point the vapor is said to be saturated, and the
pressure of that vapor (usually expressed in mmHg) is called the
saturated vapor pressure. Since the molecular kinetic energy is
greater at higher temperature, more molecules can escape the
surface and the saturated vapor pressure is correspondingly higher.
If the liquid is open to the air, then the vapor pressure is seen as a
partial pressure along with the other constituents of the air. The
temperature at which the vapor pressure is equal to the atmospheric
pressure is called the boiling point.
Vapour pressure of solution
Video
Vapour pressure of solution
The vapour pressure of solution is found to be less than that of
the pure solvent.
Raoult’s law for Binary solutions of volatile liquids
At a given temperature, for a solution of volatile liquids, the
partial pressure of each component is equal to the product of the
vapour pressure of the pure component and its mole fraction.
Suppose a binary solution consists of two volatile liquids A and
B. If and
are the partial vapour pressure of the two lquids and
a
are their mole fractions in solution, then
Raoult’s law for solutions containing non-volatile
solutes
Vapour pressure of the solution=Vapour pressure of the
solvent in the solution
If is the vapour pressure of the solvent over a solution
containing non-volatile solute and is its mole fraction
then according to Raolt’s law,
or
At a given temperature , the vapour pressure of a
solution containing non-volatile solute is directly
proportional to the mole fraction of the solvent
Collogative properties
The dilute solutions of non-volatile solutes exhibit certain
characteristic properties which don’t depend upon the nature of
the solute but depend only on the number of particles of the
solute, on the molar concentration of the solute. These are
called colligative properties. Thus
1. Relative lowering in vapour pressure
2. Elevation in boiling point
3. Depression in freezing point
4. Osmotic pressure
This mean that if two solutions contain equal number of solute
particles of A and B then the two solutions will have same
colligative properties
pressure
Dependence general presser &
partial pressure according to the
concentration
Concentration
Diagram of the positive (А) & the
negative (B) divergences from A
Raoult’s law
А
Concentration
В
А
А
Concentration
В
Dependence of the vapor pressure
according to the nature of composition
(for non ideal solutions)
Vapour
Liquid
Concentration
The curves of boiling (А) &
freezing (B) for a pure water &
solution (solvent)
Boiling of
Аsolution
tº
tº
Б
Freezing of water
0
100
Boiling of water
Freezing of solution
Time
time
The relative lowering in vapour pressure of an
ideal solution containing the non-volatile
solute is equal to the mole fraction of the solute
at a given temperature.
where A is a solvent, B is a solute
Elevation in boiling point
The boiling point of a liquid is the temperature at which its
vapour pressure becomes equal to the atmospheric pressure.
The boiling point of the solution is always higher than that
of the pure solvent. The different in the boiling points of the
solution
and pure solvent
is called the elevation in
boiling point
It has been found out experimentally that the elevation in
the boiling point of a solution is proportional to the
molality concentration of the solution
where
is called molal elevation constant or
ebullioscopicconstant
Depression in freezing point
The freezing point is the temperature a which the solid and
the liquid states of the substance have the same vapour
pressure. The freezing point of the solution is always
lower than that of the pure solvent.
where
is the molal depression constant or molal
cryoscopic constant
Determination of Molar mass
Conditions for getting accurate value of
molar mass
1. The solute must be non-volatile.
2. The solution must be dilute, concentration
of the solute in the solution should not be
more than 5 %
3. The solute should not undergo either
dissociation or association in the solution.
Osmotic pressure
OSMOSIS. It is the movement of water across a semipermeable membrane from an area of high water potential
(low solute concentration) to an area of low water potential
(high solute concentration). It is a physical process in
which a solvent moves, without input of energy, across a
semi-permeable membrane (permeable to the solvent, but
not the solute) separating two solutions of different
concentrations
or
Osmosis is the phenomenon of the flow of solvent through
a semi-permeable membrane from pure solvent to the
solution.
Osmosis can also take place between the solutions of
different concentrations. In such cases, the solvent
molecules move from the solution of low solute
concentration to that of higher solute concentration.
Difference between osmosis and diffusion
Osmotic pressure depends upon the
molar concentration of solution
Van’t Hoff observed that for dilute solutions, the
osmotic pressure is given as:
Determination of Molar Mass from
Osmotic Pressure
If two solutions have same osmotic pressure are
called isotonic solutions or isoosmotic solutions
If a solution has more osmotic pressure than some other
solutrion , it is called hypertonic
On the other hand, a solution having less osmosis pressure
than the other solution is called hypotonic
To note that a 0,9% solution of sodiun chloride (known as
saline water) is isotonic with human blood corpuscles. In
this solution, the corpuscles neither swell nor shrink.
Therefore, the medicines are mixed with saline water before
being injected into the veins.
5% NaCl solution is hypertonic solution and when red blood
cells are placed in this solution, water comes out of the cells
and they shrink
On the other hand, when red blood cells are placed in distilled
water (hypotonic solution), water flows into the cells and
they swell or burst
7. Molecules of certain substances dissociate in a solvent to
give two or more particles. For example:
Consequently, the total number of particles increases in
solution and, therefore, the colligative properties of such
solutions will be large.
Van’t Hoff factor ‘i’ to express the extent of association or
dissociation of solutes in solution. It is the ratio of the
normal and observed molar masses of the solute…
In case of association, observed molar mass being more than
the normal, the factor ‘i’ has a value less than 1. But in
case of dissociation, the Van’t Hoff factor is more than 1
because the observed molar mass has a lesser value.
In case of solutes which do not undergo any association or
dissociation in a solvent. Van’t Hoff factor ‘i’ will be equal
to 1 because the observed and normal molar masses will be
same.
Since the molar mass are inversely proportional to the
colligative property, Van’t Hoff factor may also be
expressed as:
i = Observed value of colligative property/Normal value of
colligative propert
Inclusion of Van’t Hoff (i) modifies the equation for
colligative properties as follows: