Name; Hasnain Nawaz Surname : Shaikh ROLL NO: 16 CH 42 B.E: Chemical Engineering (In Progress). Mehran University of Engineering and Technology Jamshore, ISO 9001 Certified.

1. Topic: RAOULT’S LAW
ROLL #16 CH 42 HASNAIN NAWAZ
PHYSICAL & ANALYTICAL CHEMISTRY

2. CONTENTS OF RAOULT’S LAW
 Introduction
 Definition
 Explanation.
 Limitation of Raoult’s Law
 Colligative Properties.
 Lowering of Vapor Pressure.
 Elevation of Boiling Point.
 Depression of Freezing Point.

3. Introduction
 Presented in1887,
 By a French Chemist,
 François-Marie Raoult

4. Definition
 The Law states that,
“The vapor pressure of solution (Ps) is directly proportional
to the mole fraction of solvent (Xsolvent).
Mathematically,
Psolution ∝ Xsolvent
Psolution = K × Xsolvent Here “Ps=P=K”
Psolution = Psolvent × Xsolvent

5. Graphical Representation
 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

6. Vapor and Vapor Pressure
 Vapor is the liquid molecule 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.

7. Vapor Pressure of Solutions
 In a closed container at constant
temperature an equilibrium
vapor pressure is established.
 Here the Dynamic Equilibrium is
established. The picture on the
left 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.
Initial Final

8. Vapor Pressure of Solutions
 The vapor pressure of a liquid in Solution and in Pure solvent is
much different.
 The vapor pressure of pure solvent is decreased when the non-
volatile solute particles are dissolved in it.
 The solute decreases the number of solvent molecules per unit
volume lowering the tendency for the molecules to escape into
vapor.

9. Limitation of Raoult’s Law
 The Solution which obeys Raolt’s Law is called Ideal Solution. However the Real Solution deviates
from Raolt’s Law.
 Raolt’s Law is only applicable on the Dilute or Less Concentrated solutions.

10. 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

11. Negative and Positive Deviation
 Negative Deviation: Positive Deviation
If the vapor pressure of a Positive Deviation is when the
mixture is lower than cohesive forces become
expected from Raoult's law, dominant over adhesive ones.
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.

12. Colligative Properties
Definition:
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,
 Nature of Substance
 Chemical Reactivity of Substance.

13.  When the solute is dissolved into a dilute solution,
Following properties are exhibited.

14. 1. 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 𝑃 > 𝑃𝑠
 The Relative Lowering of Vapor Pressure
𝑃−𝑃𝑠
𝑃

15. 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.
Then,
 The Relative Lowering of Vapor Pressure
𝑃−𝑃𝑠
𝑃

16. Relative Lowering of Vapor Pressure
“The relative lowering of vapor pressure of solution (
𝑃−𝑃𝑠
𝑃
)is
equal to the mole fraction of solute (Xsolute).
Mathematically,
𝑃−𝑃𝑠
𝑃
= Xsolute
𝑃−𝑃𝑠
𝑃
=
𝑛
𝑛+𝑁
Here “P>Ps”
NOTE:- The Relative Lowering Of Vapor Pressure
is referred to the all values relative to the
Vapor pressure of Pure Solvent.

17. 2. 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.

18. Example:
Type equation here.
𝑇 > 𝑇𝑏
ΔTb= T - Tb

19. Boiling Point Elevation

20. Boiling Point Elevation
 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.

21. 3. 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 solvent and is directly proportional to the molality of the
solute.

22. Example:
 When the salt is added to the water, the water freezes before its freezing
point. And as a result the Freezing Point Depressed.
 Salt water has a lower vapor pressure than pure water

23. 𝑇𝑓 > 𝑇
ΔTf = Tf - T
Freezing Point Depression

24. Freezing Point Depression
 ∆ 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

25. THANKS FOR BEARING ME SO.!! :-P