Boiling Point Elevation Calculator

Boiling point elevation is a colligative property, meaning it depends on how many particles are dissolved rather than on what they are. A mole of sugar and a mole of glycerol raise the boiling point by the same amount, because the solvent cannot tell them apart. When a non-volatile solute is dissolved, the boiling point of the solution sits above that of the pure solvent.

The formula: ΔTb = Kb × m × i

Where:

• ΔTb = Boiling point elevation (in Kelvin or °C, same magnitude)
• Kb = Ebullioscopic constant (boiling point elevation constant) of the solvent
• m = Molality of the solution (moles of solute per kilogram of solvent)
• i = Van’t Hoff factor (number of particles the solute dissociates into)

Molality formula: m = moles of solute / kilograms of solvent = (mass of solute / molar mass of solute) / mass of solvent (in kg)

Van’t Hoff factor (i):

• For non-electrolytes (sugar, glucose, urea): i = 1 (no dissociation)
• For NaCl (sodium chloride): i = 2 (Na⁺ and Cl⁻)
• For CaCl₂ (calcium chloride): i = 3 (Ca²⁺ and 2 Cl⁻)
• For MgSO₄: i ≈ 1.3, well below the 2 you would predict, because the doubly charged ions pair up in solution instead of moving independently

That last one is the useful warning. The whole-number factors above are ideal values that assume complete dissociation, and real solutions fall short of them, more so as concentration rises. Measured i for NaCl is around 1.9 at 0.1 molal rather than 2.0.

The dropdown here offers the four ideal values, which is the right starting point and is what a textbook problem expects. Just know which way the error runs: a real ionic solution dissociates less than the whole number says, so this page will slightly over-predict the elevation. The van ’t Hoff factor calculator works out the measured factor from an observed elevation, which is the other direction of the same relationship.

Kb values for common solvents:

Practical example: Salted pasta water: Adding 10g of NaCl (molar mass 58.44 g/mol) to 1 liter of water (1 kg):

• Moles of NaCl = 10 / 58.44 = 0.171 mol
• Molality = 0.171 mol / 1 kg = 0.171 mol/kg
• ΔTb = 0.512 × 0.171 × 2 = 0.175°C

Result: salted pasta water boils at about 100.17°C. That is a negligible difference, and it is in the wrong direction for the usual kitchen claim. Salting water does not make it boil faster; it raises the boiling point slightly, so the pot takes marginally longer. Salt the pasta water for flavor and for nothing else.

Where this formula stops working

ΔTb = Kb × m × i is a dilute-solution approximation, and it drifts once molality climbs much past about 1 mol/kg. The solution stops behaving ideally, and the whole-number van ’t Hoff factor stops being a fair description of what the ions are doing. Saturated salt water is roughly 6 molal. The formula predicts about 6°C of elevation there, while saturated brine is usually quoted as boiling near 108.7°C, so the real elevation is closer to 9°C. The error is not small, and it is in the direction people do not expect. Use this for the dilute end and treat anything above a molality of 1 as a rough guide.

The related freezing point depression uses the same shape with a different constant, and for water Kf is 1.86, more than three times Kb. That is why salt on an icy road does something visible and salt in a pot of water does not.