Vapour Pressure Calculator

Dissolving something in a liquid lowers how readily that liquid evaporates, and Raoult's law makes the effect precise, so this calculator works out the vapour pressure of a solution that contains a non-volatile solute. Enter the moles of solvent, the moles of solute, and the vapour pressure of the pure solvent, and it returns the mole fraction of the solvent and the vapour pressure of the solution, updating as you type. The principle is intuitive once you picture the surface of the liquid: evaporation happens when solvent molecules at the surface escape into the vapour, and when some of those surface spots are occupied by dissolved solute particles, fewer solvent molecules can leave, so the vapour pressure drops. Raoult's law captures this exactly by saying the solution's vapour pressure is the pure solvent's vapour pressure scaled down by the mole fraction of the solvent, the share of all the particles that are solvent. Add more solute and the mole fraction of solvent falls, and the vapour pressure falls with it in direct proportion. This lowering of vapour pressure is one of the colligative properties, a family of effects, alongside boiling-point elevation and freezing-point depression, that depend only on how many solute particles are present and not on their chemical identity, which is why salt or sugar, antifreeze in a radiator, and the saltiness of the sea all change how their solutions evaporate, boil and freeze. That makes the tool genuinely useful for chemistry students learning about solutions, mole fractions and colligative properties and checking homework, and for anyone needing the vapour pressure of a simple solution. Keep the pure solvent pressure in whatever unit you like; the answer comes out in the same one. The formula and a worked example are explained clearly below.

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Assumes a non-volatile solute. The solute adds particles but does not evaporate itself.