How this calculator works
A nonvolatile solute lowers the solvent mole fraction and therefore its equilibrium vapour pressure in an ideal solution. The pure-solvent pressure must refer to the same temperature. Count dissolved particles appropriately; a dissociating electrolyte cannot simply use formula-unit moles without considering its solution behaviour. The chart spans solvent mole fractions from zero to one and uses kPa. Volatile second components and non-ideal activity coefficients are outside this one-solvent model.
Formula
P = xsolvent Ppure; xsolvent = nsolvent/(nsolvent + nsolute); pressure lowering = (1 - xsolvent) Ppure.
Worked example
With 3 mol solvent and 1 mol nonvolatile solute, x = 0.75. A pure pressure of 10 kPa becomes 7.5 kPa. A 10 kPa solution over a 20 kPa pure solvent has x = 0.5.
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Frequently asked questions
Does this apply to two volatile liquids?
Each volatile component contributes its own partial pressure. This page assumes the solute contribution is negligible.
Why keep temperature fixed?
The pure-liquid vapour pressure depends strongly on temperature.
What if pressure exceeds the pure-solvent value?
That is incompatible with this nonvolatile-solute ideal model and is rejected.