Raoult's Law states that the vapor pressure of a solution is directly proportional to the mole fraction of solvent. It describes how solute particles affect the vapor pressure, boiling point, and freezing point of a solution, including key concepts like colligative properties and limitations regarding ideal solutions. The document outlines various properties such as lowering of vapor pressure, boiling point elevation, and freezing point depression, with formulas and examples illustrating these phenomena.

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