What is Raoult's Law?

Raoult's Law is a fundamental principle in physical chemistry that describes how the vapor pressure of a solvent changes when a solute is added to it. Established by François-Marie Raoult in 1887, the law states that the partial vapor pressure of each component of an ideal mixture of liquids is equal to the vapor pressure of the pure component multiplied by its mole fraction in the mixture.

In simpler terms, if you dissolve a non-volatile solute (like salt or sugar) into a solvent (like water), the vapor pressure of the resulting solution will be lower than that of the pure solvent. This happens because the solute particles occupy space at the surface, reducing the number of solvent molecules that can escape into the gas phase.

The Raoult's Law Formula

The mathematical expression for a solution containing a non-volatile solute is:

P_solution = X_solvent × P°_solvent

• P_solution: The vapor pressure of the solution.
• X_solvent: The mole fraction of the solvent (Moles of Solvent / Total Moles).
• P°_solvent: The vapor pressure of the pure solvent at a specific temperature.

How to Use the Calculator

Using our Raoult's Law Calculator is straightforward. Follow these steps to find the vapor pressure of your solution:

1. Enter Pure Vapor Pressure: Provide the vapor pressure value of the pure solvent. This value is usually found in chemical reference tables for specific temperatures.
2. Enter Mole Fraction: Input the mole fraction of the solvent. Remember, the mole fraction is a ratio between 0 and 1. If you have the mole fraction of the solute, subtract it from 1 to get the solvent's fraction.
3. Calculate: Click the calculate button to see the resulting vapor pressure of the solution.

FAQs about Raoult's Law

1. What is an ideal solution?
An ideal solution is one where the interactions between different molecules (solvent-solute) are identical to the interactions between similar molecules (solvent-solvent or solute-solute). Most real solutions only follow Raoult's Law at very low concentrations.

2. What causes a negative deviation from Raoult's Law?
Negative deviation occurs when the attraction between the solvent and solute molecules is stronger than the attraction in the pure substances. This makes it harder for molecules to escape, lowering the vapor pressure more than predicted.

3. Can this be used for volatile solutes?
Yes, but you must calculate the partial pressure for both components and add them together: P_total = (X_AP°_A) + (X_BP°_B).