Raoults law 2nd assignment

roults law

1. Physical Chemistry


Assignment no 2
“Raoult’s Law”




BS Chemistry 3
rd
The University of Lahore

2. Raoult’s Law:

History:
 Presented in1887,
 By a French Chemist, François-Marie Raoult


Definition:
“The vapor pressure of solution (Ps) is directly proportional to the mole
fraction of solvent (Xsolvent).”
Mathematically:
Psolution ∝ Xsolvent
Psolution = K × Xsolvent
Psolution = Psolvent × Xsolvent

Graphical Representation:

3. Raoult’s Law is a linear equation (y=mx+b)
A plot of Psolution vs mole fraction
gives a straight line with a slope
equal to Psolvent.
Psolution ∝ Xsolvent





Vapors & vapor Pressure:
 Vapors are the liquid molecules in gas form over the liquid surface.
Remove the liquid and you have a gas!
 Vapor Pressure:
At the particular temperature, the pressure acted over the substance( solid or liquid) at
which the Vapors are formed then that pressure is called Vapor Pressure.
 The formation of Vapor always in Dynamic Equilibrium, means,
 The rate at which the solid or liquid is evaporated is equal to the rate at which the
liquid is
 condensed back to its original form.
 All Solids and liquids have their own vapor pressure.

4. Vapor Pressure of Solutions:
 In a closed container at constant temperature an equilibrium vapor pressure is
established.
 Here dynamic equilibrium is established. The picture below indicates that vapor
molecules leave a solvent to dilute a solution.
 The solute decreases the number of solvent molecules per unit volume lowering
the tendency for the molecules to escape into vapor.


Limitations of Raoults law:
 The Solution which obeys Raoult’s Law is
called Ideal Solution. However the Real
Solution deviates from Raoult’s Law.

5.  Raoult’s Law is only applicable on the Dilute or Less Concentrated solutions.
 If AA Denotes Solvent-Solvent molecules, and AB denotes Solute & Solvent
molecules and γ is attractive force between molecules then,
For IDEAL SOLUTIONS,
γAA = γAB
While this is not true for,REAL SOLUTIONS,
γAA < γAB or γAA > γAB









Negative & Positive Deviation:
Negative Deviation: If the vapor pressure of a mixture is lower than expected from
Raoult's law, there is said to be a negative deviation.
In negative deviation
Adhesive forces between Like molecules is dominant Over the cohesive forces
Between like molecules.

6. Positive Deviation: Positive Deviation is when the cohesive forces become dominant
over adhesive ones.











Colligative Properties:
“Colligative Properties are those properties, observed when the non-volatile solute
particles are dissolved in the dilute solution.”
 Colligative Properties depend on No: of Particles (how many solute particles are
present as
 well as the solvent amount).
 But these properties are independent of

7.  Nature of Substance
 Chemical Reactivity of Substance
 When the solute is dissolved into a dilute solution, Following properties are
exhibited.


Lowering of Vapor Pressure:
Vapor pressure of pure solvent is decreased when the non- volatile solute is dissolved in
it.
If, P is the Vapor Pressure of pure solvent. Ps is the vapor pressure of Solution.
Then,
Lowering of Vapor Pressure =
ΔP = P − Ps
Where The Relative Lowering of Vapor Pressure

8. ?????? −??????
??????
??????



Relative lowering of Vapor Pressure:
“The lowering of vapor pressure is relative to the vapor pressure of pure solvent is
referred as a Relative Lowering of Vapor Pressure”
 The Relative Lowering of Vapor Pressure
?????? −??????
??????
??????

 The relative lowering of vapor pressure
?????? −??????
??????
??????
is equal to mole fraction of solute
(Xsolute)
 Mathematically:
?????? −??????
??????
??????
=
??????
?????? + ??????

 The Relative lowering of vapor pressure refers to the all relative values of vapor
pressure and pure solvent.

9. Boiling Point Elevation:
 Boiling occurs when the vapor pressure of a liquid equals atmospheric pressure.
 But since the vapor pressure of a solution is always lower than that of the pure
solvent, more Heat will be needed to boil the liquid.
 The quantitative relationship
which describes this behavior
looks like this:
∆ Tb = Kbm
 ∆ Tb is the change in the boiling
point.
 Kb is the "molal boiling point
constant" which is a property of
the solvent.
 m is the molality of the solute in
the solution.

Freezing Point Depression:
 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

10. solvent and is directly proportional to the molality of the solute.
 ΔTf = Tf - T





 ∆ Tf=Kfm
 Where ∆ Tf is the freezing point depression, the change in freezing point between
the pure solvent and the solution.
 Kf is the molal freezing point constant.

 Values depend on the solvent.
 m is the molality of the solute in the
solution