Calculate vapor pressure lowering.
Raoult's law states that the vapour pressure above a solution is the pure solvent's vapour pressure multiplied by the solvent's mole fraction. A non-volatile solute occupies part of the liquid surface and reduces the fraction of solvent molecules able to escape, so the vapour pressure falls in direct proportion. Electrolytes lower it more than their formula amount suggests, because each formula unit contributes several independent particles to the mole count.
Raoult's law
solvent mole fraction = solvent moles / (solvent moles + solute particles); P solution = P pure x mole fraction; lowering = P pure - P solution
Because it depends only on the number of dissolved particles, not their identity. That is also why it underlies boiling point elevation and freezing point depression, which are consequences of the same shift in the phase equilibrium.
It is exact only in the dilute limit. Real solutions deviate positively or negatively depending on whether solute-solvent interactions are weaker or stronger than solvent-solvent ones, and deviations become substantial above roughly 0.1 mole fraction solute.
Then both components contribute vapour and you need Raoult's law applied to each, summing the partial pressures. That is the basis of fractional distillation, and it is not what this calculator models.