Chemistry class 12th Raoult's law and henry law

Raoult's Law as a Special Case of Henry's Law


Raoult's Law as a Special Case of Henry's Law


1. Introduction
Raoult's Law and Henry's Law are both fundamental principles in physical chemistry that describe
the behavior of gases and liquids in equilibrium. Raoult's Law deals with the vapor pressure of a
solvent in a solution, while Henry's Law describes the solubility of a gas in a liquid. However,
Raoult's Law can be considered a special case of Henry's Law under certain conditions.


2. Statement of the Laws
- **Raoult's Law:** The partial vapor pressure of a component in an ideal solution is proportional to
its mole fraction and the vapor pressure of the pure component.
Formula: P_A = X_A * P_A^0


- **Henry's Law:** The amount of a gas dissolved in a liquid is proportional to the partial pressure of
the gas above the liquid.
Formula: C = k_H * P_g


Where:
- C is the concentration of the dissolved gas.
- k_H is Henry's Law constant.
- P_g is the partial pressure of the gas.


3. Raoult's Law as a Special Case of Henry's Law
Raoult's Law can be derived from Henry's Law when considering a volatile solute dissolved in a
liquid. If we assume the solute behaves ideally, the equilibrium between the liquid and gas phases
follows Henry's Law:


- In an ideal solution, the solute's partial pressure above the solution is proportional to its
concentration in the liquid phase.
- Henry's Law states P_solute = k_H * C_solute.
- If the solute's concentration is expressed in terms of its mole fraction (X_solute), then P_solute =
k_H * X_solute.
- In an ideal system, Henry's Law constant (k_H) becomes the vapor pressure of the pure solute
(P_solute^0), leading to Raoult's Law: P_solute = X_solute * P_solute^0.