Raoult's Law Calculator

At a glance:Raoult's law (proposed by the French chemist François-Marie Raoult in 1887) is a basic law describing the vapor pressure of solutions. It states that for an ideal solution, the vapor pressure P of the solvent above the solution equals the pure-solvent vapor pressure P° times the mole fraction of the solvent x_solvent: P = x_solvent × P°. When you add a 'non-volatile solute' (one that itself produces almost no vapor, like sugar or salt dissolved in water), the solvent mole fraction x_solvent becomes less than 1, so the solution vapor pressure P is necessarily lower than the pure solvent's P° — this phenomenon is called 'vapor-pressure lowering'. Since x_solvent + x_solute = 1, the lowering can be written ΔP = P° − P = P° × (1 − x_solvent) = x_solute × P°. Thus the lowering depends only on the solute mole fraction, not on what the solute is — exactly the hallmark of a 'colligative property'. Using this tool's default: solute mole fraction x_solute = 0.1, pure-solvent vapor pressure P° = 100 kPa, then ΔP = 0.1 × 100 = 10 kPa and the solution vapor pressure P = 100 − 10 = 90 kPa. Microscopically, vapor-pressure lowering happens because solute particles dispersed in the solvent occupy part of the liquid surface and lower the escaping tendency of solvent molecules (lowering the chemical potential), so fewer solvent molecules evaporate per unit time and the equilibrium vapor pressure is lower. Vapor-pressure lowering is the basis for understanding the other two colligative properties: because the vapor pressure is pulled down, the solution must be heated to a higher temperature for its vapor pressure to reach atmospheric pressure, giving 'boiling-point elevation'; similarly the solid–liquid equilibrium temperature is pulled down, giving 'freezing-point depression'. Applications of Raoult's law: first, computing the vapor pressure of solutions with non-volatile solutes (sugar water, salt water). Second, as a criterion for ideal solutions — real solutions that deviate positively or negatively reveal differences in intermolecular forces. Third, determining solute molar mass (back-calculate mole fraction from the lowering, then the molar mass). Fourth, for volatile binary mixtures, each component obeys Raoult's law, allowing computation of total pressure and composition (distillation principle). Notes: first, this formula applies to 'non-volatile solute' ideal dilute solutions; concentrated solutions deviate. Second, use mole fraction, not mass fraction, and x_solute between 0 and 1. Third, P° is the vapor pressure of the pure solvent 'at that temperature'; change temperature, change P°. Fourth, if the solute is an electrolyte, count the total dissociated particles (multiply by the van't Hoff factor).