Colligative properties such as boiling point elevation, freezing point depression, and osmotic pressure depend only on the number of solute particles in solution, not their identity. The document discusses these properties and how they are affected when solutes dissolve. It explains that boiling point is elevated and freezing point is depressed because solutes lower vapor pressure, requiring more energy for phase changes. Osmotic pressure arises from differences in solute concentration across a semipermeable membrane. The van't Hoff factor accounts for changes in observed properties when solutes dissociate or associate in solution.
2. 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
depression of freezing point of solvent in
solution
Osmotic pressure
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3. BOILING POINT ELEVATION
BOILING POINT
Boiling point is definded as the temperature at
which the vapour pressure of liquid becomes
equal to the atmospheric pressure .
Boiling point is a characteristic property of
liquids and is criterion to check the purity of
liquids.
It increases with in external pressure . Liquids
having grater intermolecular forces have high
boiling point.
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4. ELEVATION OF BOILING POINT
It is known that a liquid boils only when its
vapour pressure becomes equal to the
atmospheric pressure. But when a non-
volatile solute is added to the solvent,the
vapour pressure is lowered.
In order to boil the solution, the
temperature has to be raised further so as to
make its vapour pressure equal to
atmospheric pressure.
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5. In the graph shown above, we can see
the change in vapour pressure of a pure
solvent and a solution with the increase in
temperature. Here, we can see that the two
samples, the solvent and the solution, will
attain the condition of boiling when their
vapour pressure becomes equal to the
atmospheric pressure. We can see here,
the pure solvent attains this condition at a
temperature equal to Tb° whereas, for the
solution to attain this pressure, the
temperature of the sample needs to be
increased by a small amount equal to ΔTb,
such that,
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6. Here the value ΔTb is known as the elevation in boiling
point. The value of ΔTb for a solution depends upon the number of
molecules of the non-volatile solute. It has been shown
experimentally that the elevation in boiling point is directly
proportional to the molal concentration of the solute in a solution.
T is the change in boiling point of the solvent,
Kb is the molal boiling point elevation constant, and
m is the molal concentration of the solute in the solution.
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7. Here molarity is the number of moles of solute
dissolved in 1 Kg of solvent and the contast of
proportionality,Kb is called Boiling point
elevation constant .
The unit of Kb is K kg mol-1
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8. FREEZING POINT DEPRESSION
FREEZING POINT
The temperature at
which liquid state is converts
into a solid state. It is
opposite to melting point.
Pressure increases freezing
point also increases.
Hence freezing point of a
liquid is a temperture at
which the vapour pressure of
soild is equal to the vapour
pressure of liquid.
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9. DEPRESSION OF FREEZING POINT
Freezing point depression is a colligative property
observed in solutions that results from the
introduction of solute molecules to a solvent.
The freezing points of solutions are all lower
than that of the pure solvent and is directly
proportional to the molality of the solute.
ΔTf=Tf°(solvent)−Tf(solution)
ΔTf is the depression in freezing point
Tf° is the freezing point of the solvent,
Tf is the freezing point of the solution
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10. FREEZING POINT DEPRESSION GRAPH
• The figure below shows the phase diagram
for a pure solvent and how it changes when a
solute is added to it. The solute lowers the
vapor pressure of the solvent resulting in a
lowering of the freezing point of the solution
compared to the solvent. The freezing point
depression is the difference in temperature
between the freezing point of the pure solvent
and that of the solution. On the graph, the
freezing point depression is represented by .
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11. • When a pure solvent freezes, its particles
become more ordered as the intermolecular
forces that operate between the molecules
become permanent. In the case of water, the
hydrogen bonds make the hexagonally-shaped
network of molecules that characterizes the
structure of ice. By dissolving a solute into the
liquid solvent, this ordering process is
disrupted. As a result, more energy must be
removed from the solution in order to freeze it,
and the freezing point of the solution is lower
than that of the pure solvent.
• The magnitude of the freezing point depression
is directly proportional to the molality of the
solution. The equation is
ΔTf= Kf m
ΔTf is freezing-point depression
Kf is called the molal freezing-point
depression constant
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12. Depression Of Freezing-point And
Molar Mass Of Solute
Let W2 Grams Of Solute With Molar Mass
M2is Dissolved In W1 Gram Of Solvent , The
Molality (M) Is Given By
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13. OSMASIS
• Osmosis is the movement of water or other
solvent through a plasma membrane from a
region of low solute concentration to a region
of high solute concentration. Osmosis is
passive transport, meaning it does not require
energy to be applied
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14. • A Technique for purifying water, in which
pressure is applied to force liquid through a
semipermeable membrane in the opposite
direction to that in normal osmosis.
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15. SEMI PERMEABLE MEMBRANE
• A semipermeable membrane is a
barrier that will only allow some
molecules to pass through while
blocking the passage of other
molecules. A semipermeable
barrier essentially acts as a filter.
• Different types of
semipermeable membranes can
block out different sized
molecules. A semipermeable
membrane can be made out of
biological or synthetic material.
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19. Isotonic: Solutions have equal concentration of solute, and so
equal osmotic pressure.
Hypertonic: Solution with higher concentration of solute.
Hypotonic: Solution with lower concentration of solute.
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20. LAW OF OSMOTIC PRESSURE
Boyle-van’t Hoff Law
Vant Hoff Charles Law
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21. Boyle-van’t Hoff Law
Osmotic pressure (p) of a solution at a constant
temperature is directly proportional to its
concentration C (i.e., moles per liter)
π∝C
π∝n/v
where n is the number of moles of solute
present in volume V liters of solution.
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22. Vant Hoff Charles Law
• For a solution of fixed concentration, the
osmotic pressure (p) of a solution is directly
proportional to its absolute temperature (T).
π∝T
π∝ nT/v
The constant of proportionality also turns
out to be the same as gas constant R. Thus,
π = nRT/v
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23. • Desalination by reverse osmosis. Oceans hold about 97 percent of
Earth's water supply, but their high salt content makes them
unusable for drinking or agriculture. Salt can be removed by placing
seawater in contact with a semipermeable membrane, then
subjecting it to great pressure. Under these conditions, reverse
osmosis occurs, by which pressure is used to push water from a
more concentrated solution to a less concentrated solution. The
process is just the reverse of the normal process of osmosis.
In desalination, reverse osmosis is used to push water mole-cules
out of seawater into a reservoir of pure wate
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27. ASSOCIATION OF SOLUTES
• some solute molecules start to associate inside
the solution. This means now there are less
number of solute particles in the solution. As
colligative properties vary with solute particles in
the solution, they will decrease along with the
solute particles. As colligative properties are
inversely proportional to the molecular mass of
solute, we get a higher molar mass of the solute.
• For Example : acetic acid ( CH3COOH) associate in
solution to form a dimer due to hydrogen
bonding.
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28. Van’t Hoff’s Factor
The van 't Hoff factor i (named after Dutch
chemist Jacobus Henricus van 't Hoff) is a
measure of the effect of a solute upon colligative
properties such as osmotic pressure, relative
lowering in vapor pressure, boiling-point
elevation and freezing-point depression.
its as definded as the ratio of observed colligative
prperty produced by a given concentration of
electrolytes solution to the prpperty obseved for
the same concentration of non electrolyte
solutions
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29. 30
The Van’t Hoff Factor is denoted by letter ‘i’.
Van’t Hoff defined this factor as follows:
30. 31
Degree of dissociation (a): It is defined as the
fraction of total molecules which dissociate
into simpler molecules or ions.
m= number of particles in solution
Degree of association (a): It is defined as the
fraction of the total number of molecules
which associate or combine together
resulting in the formation of a bigger
molecules.
• ; m = number of particles in solution.
