Boiling Point Elevation Calculator

Dissolving something in a liquid makes that liquid harder to boil. This calculator tells you by how much, and — just as usefully — it will read the answer backwards so you can work out how many particles your own solute produced once it was dissolved.

You need three things to predict it: how strong the effect is for that particular liquid (the ebullioscopic constant), how concentrated your solution is, and how many separate particles each formula unit broke into. Pick your solvent from the menu and the first two are filled in for you, because they are properties of the liquid rather than of your experiment.

The people who reach for this are usually doing one of three jobs: checking a recipe that relies on salt to hold a boil back, working out at what concentration a solution stops behaving ideally, or using a measured boiling point to interrogate an unknown solute. If your solvent boils somewhere other than one atmosphere — a high-altitude kitchen, a reduced-pressure distillation — start from the boiling point calculator to get the pure-solvent number first, then type it in here.

Pick Water, set the van't Hoff factor, and read both answers off. The three tabulated boxes lock themselves, which is the point: they describe the solvent, so letting them drift would quietly contradict the substance you selected.

The van't Hoff factor is the one number people most often have to guess. This is the route that removes the guess: enter the boiling point you actually measured, and the tool solves for the particle count instead of asking you to supply it.

Say your solution is 1 molal in water and you found it boiled at 100.30 °C. Leave the van't Hoff factor alone and fill in the solution boiling point instead:

A number well below 2 is information, not an error. It says the solute did not deliver two independent particles per formula unit in this solution at this concentration — ion pairs holding the two halves together, or association into clusters, both of which pull the measured factor down below the textbook one. The same solute at a different concentration can land somewhere else entirely, which is exactly why this calculator never picks the value for you.

Two statements do all the work. The first is the colligative relation itself; the second just says where that lands on the temperature scale.

Both can be run in reverse, which is why any box can be the one you type into. Give an elevation and the ebullioscopic constant and the molality, and the tool solves for the particle count; give the two boiling points and it solves for the elevation.

Both mass menus can be on any scale

The ebullioscopic constant is quoted per unit of solvent mass and so is the molality, but they sit on opposite sides of that mass, so the two numbers move in opposite directions when you change scale:

A pound of solvent holds more moles than a kilogram does, which is why the molality figure shrinks — but it also makes a mole in a pound a more concentrated solution, which is why the constant per pound grows. The two displayed numbers still multiply to the same 1.536 as the kilogram pair does, which is what lets you check the arithmetic on screen. The calculator normalises both independently, so the answer is right even on mismatched scales.

A rise is not a level

The elevation carries no 273.15 offset, because it is a difference rather than a temperature. 1.536 °C of elevation is 1.536 K, and 2.7648 °F — the same rise, three different labels. The two boiling-point boxes, by contrast, are levels and do carry the offset, which is what makes them add correctly.

These are the figures the calculator ships with. Water's boiling point is the familiar 100 °C; the published value is 373.17 ± 0.04 K on the NIST Chemistry WebBook, which is the same number written more precisely. Note how far the constants spread: water sits at the bottom because water molecules cling to each other unusually tightly, and that same strong cohesion is why water has such a high normal boiling point for its small mass.

This is a dilute-solution estimating tool. It assumes the solute does not evaporate and that the mixture still obeys Raoult's law to first order, and both assumptions fail before the arithmetic does.

• Dilute only. The relation is a first-order expansion. Every tabulated constant assumes a solution thin enough for that expansion to hold.
• A non-volatile solute. If your solute boils appreciably below the solvent — alcohol in water, say — it leaves with the vapour and raises the vapour pressure instead of lowering it, which inverts the sign of the effect.
• The factor is not constant. Dissociation and association both vary with concentration, which is why the tool leaves $i$ free rather than picking a textbook number for you.
• The tabs are handbook figures. They are printed to two or three significant figures, and the calculator keeps each one exactly as published rather than implying precision the source does not have.
• Both factors must be positive. A zero or negative ebullioscopic constant, molality or van't Hoff factor is refused rather than quietly accepted, and the tool flags a molality or an elevation beyond anything a real liquid can hold instead of returning a confident number.