Raoult’s Law:

Raoult’s Law: Raoult's law states: the vapor pressure of an ideal solution is dependent on the vapor pressure of each chemical component and the mole fraction of the component present in the solution. For ideal solution, the equilibrium partial pressure P* of a constituent at a fixed temperature its vaporpressure (P) when pure at this temperature & its mole fraction in equilibriumliquid. PA* = PA ⋅ x PB* =PB (1-x) PA*, PB* =equilibrium partial pressure of A & B respectively PA, PB = vapor pressure of pure A & pure B at distillation temperature. x = mole fraction of more volatile component (A) in liquid 1-x = mole fraction of less volatile component (B) in liquid Using the example of a solution of two liquids, A and B, if no other gases are present, then the total vapor pressure Pt above the solution is equal to the weighted sum of the "pure" vapor pressures of the two components, PA and PB. If the vapor phase is also ideal, then Pt = PA* + PB* = PA ⋅ x + PB (1-x) Consequently, as the number of components in a solution increases, the individual vapor pressures decrease, since the mole fraction of each component decreases with each additional component. If a pure solute which has zero vapor pressure (it will not evaporate) is dissolved in a solvent, the vapor pressure of the final solution will be lower than that of the pure solvent. This law is strictly valid only under the assumption that the chemical interaction between the two liquids is equal to the bonding within the liquids: the conditions of an ideal solution. Therefore, comparing actual measured vapor pressures to predicted values from Raoult's law allows information about the relative strength of bonding between liquids to be obtained. If the measured value of vapor pressure is less than the predicted value, fewer (or less) molecules have left the solution than expected. This is put down to the strength of bonding between the liquids being greater than the bonding within the individual liquids, so very fewer (less) molecules have enough energy to leave the solution. Conversely, if thevapor pressure is greater than the predicted value more molecules have left the solution than expected, due to the bonding between the liquids being less strong than the bonding within each. The vapor pressure and composition in equilibrium with a solution can yield valuable information regarding thethermodynamic properties of the liquids involved. Raoult’s law relates the vapor pressure of components to the composition of the solution. The law assumes ideal behavior. It gives a simple picture of the situation just as the ideal gas law does. The ideal gas law is very useful as a limiting law. As the interactive forces betweenmolecules and the volume of the molecules approaches zero, so the behavior of gases approach the behavior of the ideal gas. Raoult’s law is similar in that it assumes that the physical properties of the components are identical. The more similar the components are, the more their behavior approaches that described by Raoult’s law. For example, if the two components differ only in isotopic content, then the vapor pressure of each component will be equal to the vapor pressure of the pure substance PA times the mole fraction in the solution. This is Raoult’s law.