What is Raoult's Law and when is it applicable?

Raoult's Law states that the partial pressure of a component in a vapor mixture equals the product of its mole fraction in the liquid phase and its vapor pressure at the system temperature: Pi = xi * Pi_sat. It applies to ideal solutions where molecules have similar sizes and intermolecular forces. Real solutions often deviate from Raoult's Law, requiring activity coefficients for accurate calculations.