Solution - class 12

Solution – Class 12
Prepared by- Debadatta mahapatra

Vapour Pressure of Pure Liquid and Solution
Vapour pressure: When a liquid is taken in a closed vessel, a part of the liquid evaporates and its vapours
occupy the available empty space.
 An equilibrium is established between vapour phase and liquid phase and the pressure that its vapour exert is
termed as vapour pressure. Thus, vapour pressureof a liquid may be defined as the pressure exerted by the
vapours above the liquid surface in equilibrium with the liquid phase at a given temperature.
The vapour pressure of a liquid depends on the following factors:
Nature of the liquid: Liquids having weak intermolecular forces are volatile and therefore have greater
vapour pressure.
Temperature: Vapour pressure of a liquid increases with increase in temperature. This is because with
increase in temperature, the kinetic energy of the molecules increases and therefore large number of
molecules are available for escaping from the surface of the liquid.

Vapour pressure of liquid–liquid solution:
Raoult’s Law for solutions of volatile liquids: It states that for a
solution of volatile liquids the partial pressure of each component of the
solution is directly proportional to its mole fraction present in a solution.
Mathematically,

Vapour pressure of solutions of solids in liquids
Raoult’s law for a solution containing a non-volatile
solute and volatile solvent: It states that the relative lowering
of vapour pressure is equal to mole fractions of solute which is
non-volatile.

Ideal and Non-Ideal Solutions
(a) Ideal solution: A solution is called an ideal solution if
it obeys Raoult’s law over a wide range of concentration at a specified
temperature.
(B) Non-ideal solution: A solution which does not obey
Raoult’s law for all concentrations is called a non-ideal solution

Vapour pressure diagram for
an ideal solution
(ii) The solution of A and B liquids will be ideal if A and B have
similar structures and polarity.
Methanol and ethanol have the same functional group and
almost same polarity and therefore , form ideal solutions

Vapour pressure diagram showing
positive deviation
Vapour pressure diagram showing
negative deviation

Characteristics of a Solution Showing Negative
Deviation
Some examples of the solution showing negative deviations are
(i) HNO3 and water
(ii) Chloroform and acetone
(iii) Acetic acid and pyridine
(iv) Hydrochloric acid and water
Some examples of the solution exhibiting positive deviations are:
(i) Ethyl alcohol and water
(ii) Acetone and carbon disulphide
(iii) Carbon tetrachloride and benzene
(iv) Acetone and benzene

Characteristics of Ideal and Non-ideal Solutions

Azeotropes or Azeotropic mixture: Azeotropes are binary mixtures having the same composition in
liquid and vapor phase and boil at a constant temperature.
(i) Minimum boiling azeotropes: These are the binary
mixtures whose boiling point is less than either
of the two components. The non-ideal solutions which
show a large positive deviation from Raoult’s
law form minimum boiling azeotrope at a specific
composition, e.g., a mixture of 94.5% ethyl alcohol
and 4.5% water by volume.
(ii) Maximum boiling azeotropes: These are the binary
mixtures whose boiling point is more than either of the two
components. The solutions that show large negative
deviation from Raoults’s law form maximum boiling
azeotrope at a specific composition, e.g., a mixture of 68%
HNO3 and 32% of H2O by mass
Types of Azeotropes:

Colligative Properties
Those properties which depend on the number of solute particles (molecules,
atoms or ions) but not upon their nature are called colligative properties. The
following are the colligative properties:
(a) Relative lowering of vapor pressure of the solvent,
(b) Elevation of boiling point of the solvent,
(c) Depression of freezing point of the solvent,
(d) Osmotic pressure of the solution.

(a) Relative lowering of vapour pressure: The addition of a non-volatile solute to a volatile solvent
decreases the escaping tendency of the solvent molecules from the surface of solutions as some of
the surface area is occupied by non-volatile solute particles. According to Raoult’s law, the relative
lowering of vapour pressure is equal to mole fraction of solute.

(b) Elevation of boiling point: Boiling point is
the temperature at which the vapour pressure
of a liquid becomes equal to the atmospheric pressure.
 When a non-volatile solute is added to a volatile solvent, the
vapour pressure of the solvent decreases.
For dilute solution, it has been found that the
elevation of boiling point is directly proportional
to the molal concentration of the solute in the
solution. Thus
Vapour pressure temperature
curves showing elevation in
boiling point of solution

(c) Depression of freezing point: Freezing point is defined as
the temperature at which the vapour pressure of a substance in its liquid phase is
equal to its vapour pressure in the solid phase.
 A solution freezes when its vapour pressure equals the vapor pressure of the
pure solid solvent.
 Whenever a non-volatile solute is added to the volatile solvent, its vapour
pressure decreases, and it would become equal to that of solid solvent at a
lower temperature.
It has been found that for a dilute solution, depression in
freezing is directly proportional to molality of the solution.
Vapour pressure temperature
curves showing depression in
freezing point of solution

 Osmosis: When a solution is separated from its solvent by a semipermeable
membrane (SPM) there is a spontaneous flow of solvent molecules from solvent
compartment to solution compartment.
The phenomenon is called osmosis. This movement of solvent is only in one direction.
In diffusion,however, movement takes place in both the directions.
Semipermeable membrane (SPM): A membrane
through which only solvent molecules can pass but
not the solute ones.
 Cellophane, parchment paper and the wall of living
cell are the examples of SPM.

ｰOsmotic pressure (p): The osmotic pressure of a
solution is the excess pressure that must be applied to
a solution to prevent osmosis, i.e., to stop the passage
of solvent molecules into it through semipermeable
Membrane.
Osmotic pressure (p) is proportional to molarity (C)of the
solution at a given temperature T.

Reverse osmosis: If a pressure larger than the
osmotic pressure is applied to the solution side, the
pure solvent (or water) flows out of the solution
through the semipermeable membrane. In this
way the direction of osmosis is reversed and so
the process is called reverse osmosis.
Isotonic solutions: Two solutions are said to be isotonic when
they exert the same osmotic pressure because they have the same
molar concentration.
 All intravenous injections must be isotonic with body fluids.
Reverse osmosis occurs when a pressure larger than
the osmotic pressure is applied to the solution

Isosmotic solutions: When two isotonic solutions are separated by a
semipermeable membrane, no osmosis occurs. The solutions are called isosmotic
solutions.
Hypotonic solutions: A solution having lower osmotic pressure than the other
solution is said to be hypotonic with respect to the other solution.
Hypertonic solution: A solution having higher osmotic pressure than the other
solution is said to be hypertonic with respect to other solution.
Plasmolysis: When the cell is placed in a hypertonic solution, the fluid comes
out of the cell due toosmosis and as a result cell material shrinks gradually. This
process is called plasmolysis.

Abnormal Molar Masses
Association: Association of molecules leads to decrease in the number of particles in
the solution resulting in a decrease in the value of colligative property.
As colligative property is inversely related to the molecular mass. Therefore, higher value
is obtained for molecular mass than normal values.
For example, when ethanoic acid is dissolved in benzene it undergoes
dimerization and shows a molecular mass of 120 (normal molecular mass is
60).

Solution - class 12

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Solution - class 12