About the Boiling Point Elevation Calculator
This boiling point elevation calculator applies the colligative-property equation ΔTb = i × Kb × m to find how much a dissolved, non-volatile solute raises the boiling point of a solvent, and what the new boiling point will be. Enter the molality directly, or let the calculator work it out from the solute mass, its molar mass and the mass of solvent.
Pick water, acetic acid, benzene, chloroform or nitrobenzene to load the ebullioscopic constant (Kb) and normal boiling point from the OpenStax Chemistry 2e table, or choose a custom solvent and type your own values. The van ’t Hoff factor i accounts for electrolytes that split into ions: about 1 for sugar, 2 for NaCl and 3 for CaCl₂ in the ideal case.
It is meant for chemistry homework, lab pre-work and curiosity questions such as how much salt really changes the boiling point of pasta water (very little). The equation is accurate for dilute solutions; at high concentrations, ion pairing makes the measured i smaller than the ideal whole number and the result less exact.
With the default inputs, the boiling point elevation δtb is 1.024 °C. Change any value above to recalculate instantly.
How to use the boiling point elevation calculator
- 1Choose the solvent, or Custom to enter your own Kb and boiling point.
- 2Enter the molality, or the solute mass, molar mass and solvent mass.
- 3Set the van 't Hoff factor for your solute.
- 4Read the boiling point elevation and the new boiling point.
Formula and method
Boiling point elevation is a colligative property: it depends on the number of dissolved particles, not on their identity. A non-volatile solute lowers the solvent’s vapour pressure, so the solution must be heated further before its vapour pressure reaches atmospheric pressure. The rise is proportional to the molality m (moles of solute per kilogram of solvent), with the solvent’s ebullioscopic constant Kb as the proportionality constant.
The van ’t Hoff factor i multiplies the molality by the number of particles each formula unit produces. The calculator adds ΔTb to the solvent’s normal boiling point at 1 atm; at other pressures, start from the boiling point at that pressure instead.
- ΔTb
- Boiling point elevation (°C or K)
- i
- van 't Hoff factor (particles per formula unit)
- Kb
- Ebullioscopic constant of the solvent (°C·kg/mol)
- m
- Molality of solute (mol/kg solvent)
Worked examples
1 mol NaCl in 1 kg water
58.44 g of NaCl is 1.00 mol; in 1.000 kg of water that is 1.00 mol/kg. With i = 2, ΔTb = 2 × 0.512 × 1.00 = 1.02 °C, so the solution boils at about 101.02 °C.
Sucrose syrup
171.15 g of sucrose is 0.500 mol; dissolved in 0.250 kg of water the molality is 2.00 mol/kg. Sugar does not ionise (i = 1), so ΔTb = 0.512 × 2.00 = 1.02 °C.
Naphthalene in benzene
Benzene has Kb = 2.53 °C·kg/mol and boils at 80.1 °C. A 0.40 mol/kg naphthalene solution raises that by 2.53 × 0.40 = 1.01 °C to 81.11 °C.
Frequently asked questions
What is boiling point elevation?+
It is the increase in a solvent’s boiling point when a non-volatile solute is dissolved in it. The solute lowers the vapour pressure, so a higher temperature is needed to boil. The size of the rise depends only on how many particles are dissolved.
What is Kb for water?+
The ebullioscopic constant of water is 0.512 °C·kg/mol (often rounded to 0.51 or 0.52). One mole of non-ionising solute in one kilogram of water raises the boiling point by about 0.51 °C.
Does adding salt to pasta water make it boil faster?+
No. A typical tablespoon of salt (about 17 g) in 4 litres of water is roughly 0.07 mol/kg, which raises the boiling point by only about 0.07 °C. Salt is for flavour, not cooking speed.
How do I find the van 't Hoff factor?+
Count the ions one formula unit releases: NaCl → Na⁺ + Cl⁻ gives 2, CaCl₂ gives 3, glucose gives 1. Measured values are a little lower in real solutions — about 1.9 for NaCl and 2.7 for MgCl₂.
Why use molality instead of molarity?+
Molality is based on the mass of solvent, which does not change with temperature. Molarity uses solution volume, which expands as it warms, so it is a poor choice for a property measured at the boiling point.