roults law

1. Physical Chemistry
Assignment no 2
“Raoult’s Law”
BS Chemistry 3rd
The University of Lahore

2. Raoult’s Law:
History:
 Presented in1887,
 By a French Chemist, François-MarieRaoult
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 pressureacted over the substance( solid or liquid) at
which the Vapors areformed then that pressureis 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 havetheir own vapor pressure.

4. Vapor Pressure of Solutions:
 In a closed container at constant temperature an equilibrium vapor pressureis
established.
 Here dynamic equilibrium is established. The picture below indicates that vapor
molecules leave a solventto 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 IdealSolution. However the Real
Solution deviates fromRaoult’s Law.

5.  Raoult’s Law is only applicable on the Dilute or Less Concentrated solutions.
 If AA Denotes Solvent-Solventmolecules, and AB denotes Solute & Solvent
molecules and γ is attractive forcebetween 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 pressureof a mixture is lower than expected from
Raoult's law, there is said to be a negative deviation.
In negative deviation
Adhesiveforces between Like molecules is dominant Over the cohesive forces
Between like molecules.

6. Positive Deviation: Positive Deviation is when the cohesiveforces 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 pressureof puresolvent 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. 𝑷 − 𝑷𝒔
𝑷
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
𝑷 −𝑷𝒔
𝑷
 The relative lowering of vapor pressure
𝑷 −𝑷𝒔
𝑷
is equal to mole fraction of solute
(Xsolute)
 Mathematically:
𝑷 − 𝑷𝒔
𝑷
=
𝒏
𝒏 + 𝑵
 The Relative lowering of vapor pressurerefers to the all relative values of vapor
pressureand pure solvent.

9. Boiling Point Elevation:
 Boiling occurs when the vapor pressureof a liquid equals atmospheric pressure.
 But since the vapor pressureof 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. solventand is directly proportionalto the molality of the solute.
 ΔTf = Tf - T
 ∆ Tf=Kfm
 Where ∆ Tf is the freezing point depression, the change in freezing point between
the puresolvent 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