Molecular Origin of Raoult’s Law

Raoult’s Law states that the partial vapor pressure of a component in an ideal solution is directly proportional to its mole fraction. On a molecular level, this behavior originates from how the intermolecular forces between solute and solvent molecules affect the ability of molecules to escape into the vapor phase.

🔬 Intermolecular Interactions in Ideal Solutions

In an ideal solution, the interactions between all types of molecules (solute-solute, solvent-solvent, and solute-solvent) are approximately equal. This equality ensures that the tendency of solvent molecules to escape from the liquid phase into the vapor phase is reduced in direct proportion to their mole fraction.

💧 Molecular Escape and Vapor Pressure

In a pure solvent, a fixed number of molecules at the surface have enough kinetic energy to overcome intermolecular forces and enter the vapor phase, creating a certain vapor pressure. When a non-volatile solute is added:

• The mole fraction of the solvent decreases.
• The number of solvent molecules at the surface decreases.
• Fewer molecules escape into the vapor phase, resulting in lower vapor pressure.

📌 Summary of Molecular Origin

The molecular origin of Raoult’s Law lies in the dilution effect of the solvent due to the presence of solute molecules. This reduces the number of solvent molecules at the surface and thereby decreases their escape tendency. In ideal solutions, where molecular interactions remain consistent across species, this leads to a proportional decrease in vapor pressure with mole fraction.