Skip to content

Boiling Point Elevation

ChemistryGas Laws and Thermodynamics
Solvent
Solute
Solute amount given as

1 for sugars and other non-electrolytes, about 2 for NaCl, 3 for CaCl2.

Boiling point of the solution (°C) —
Elevation ΔTb (°C)
—
Molality (mol/kg)
—
Kb used (°C·kg/mol)
—
Solvent
—
Boiling point (°F)
—

Boiling point elevation is ΔTb = i × Kb × m, where m is molality in mol/kg, Kb is the solvent's ebullioscopic constant and i is the van 't Hoff factor. For 1 mol of NaCl in 1 kg of water, i = 2 and Kb = 0.512, so ΔTb = 1.024 °C and the solution boils at about 101.02 °C.

About this tool

Chemistry students working colligative-property problems, and anyone checking how much a dissolved salt or sugar lifts a boiling point, need the same three numbers: molality, the solvent's Kb and the van 't Hoff factor. This calculator holds Kb and normal boiling points for seven common solvents from the OpenStax table, or takes your own Kb for anything else. Give the solute as a molality directly, or as grams of solute, its molar mass and grams of solvent, and it works out the molality first. It reports the new boiling point in °C and °F, the elevation, and each line of the arithmetic. The relation is the dilute ideal-solution limit: ion pairing and non-ideal behaviour make real elevations smaller than i × Kb × m predicts, especially above roughly 1 mol/kg.

How to use it

  1. Choose the solvent

    Pick one of the listed solvents, or Other to type a Kb and the pure solvent's boiling point.

  2. Say how the solute is given

    Choose Molality to type mol/kg directly, or Masses to enter solute grams, molar mass and solvent grams.

  3. Set the van 't Hoff factor

    Use 1 for molecular solutes and the number of ions per formula unit for strong electrolytes.

  4. Read the result

    The boiling point, the elevation and the worked lines update as you type.

Examples

0.33 m solute in benzene

Solvent
Benzene
Solute amount given as
Molality
Molality
0.33
Van 't Hoff factor i
1

Result Boiling point of the solution (°C): 80.93
Elevation ΔTb (°C): 0.835
Molality (mol/kg): 0.33
Kb used (°C·kg/mol): 2.53
Solvent: Benzene
Boiling point (°F): 177.68

  1. ΔTb = i × Kb × m = 1 × 2.53 × 0.33 = 0.8349 °C
  2. Boiling point = 80.1 + 0.8349 = 80.93 °C

Benzene's Kb of 2.53 °C·kg/mol is five times that of water, so a modest 0.33 m solution already lifts the boiling point by 0.83 °C.

58.44 g NaCl in 1 kg water

Solvent
Water
Solute amount given as
Masses
Mass of solute
58.44
Molar mass of solute
58.44
Mass of solvent
1000
Van 't Hoff factor i
2

Result Boiling point of the solution (°C): 101.02
Elevation ΔTb (°C): 1.024
Molality (mol/kg): 1
Kb used (°C·kg/mol): 0.512
Solvent: Water
Boiling point (°F): 213.84

  1. Moles of solute = 58.44 g ÷ 58.44 g/mol = 1 mol
  2. Molality m = 1 mol ÷ 1 kg = 1 mol/kg
  3. ΔTb = i × Kb × m = 2 × 0.512 × 1 = 1.024 °C
  4. Boiling point = 100 + 1.024 = 101.02 °C

One mole of NaCl splits into two ions, so i = 2 and the elevation is 2 × 0.512 × 1 = 1.024 °C.

1 m sucrose in water

Solvent
Water
Solute amount given as
Molality
Molality
1
Van 't Hoff factor i
1

Result Boiling point of the solution (°C): 100.51
Elevation ΔTb (°C): 0.512
Molality (mol/kg): 1
Kb used (°C·kg/mol): 0.512
Solvent: Water
Boiling point (°F): 212.92

  1. ΔTb = i × Kb × m = 1 × 0.512 × 1 = 0.512 °C
  2. Boiling point = 100 + 0.512 = 100.51 °C

For a non-electrolyte at 1 mol/kg the elevation equals Kb itself, 0.512 °C.

How it is calculated

ΔTb = i × Kb × m; Tb(solution) = Tb(solvent) + ΔTb; m = (mass of solute ÷ molar mass) ÷ kg of solvent

ΔTb
rise in boiling point, °C (equal in kelvin)
i
van 't Hoff factor, particles per formula unit dissolved
Kb
ebullioscopic constant of the solvent, °C·kg/mol
m
molality of the solute, mol per kg of solvent

A nonvolatile solute lowers the solvent's vapour pressure, so the liquid must be heated further before its vapour pressure reaches the surrounding pressure. For dilute solutions the rise is proportional to the number of dissolved particles per kilogram of solvent, which is why molality (not molarity) is used and why ionic solutes are multiplied by i. Kb values and normal boiling points are those in OpenStax Chemistry 2e Table 11.2, all at 1 atm.

When not to use it

  • Not for volatile solutes such as ethanol in water, which change the vapour pressure in other ways.
  • Not for concentrated solutions where the ideal-dilute assumption breaks down.
  • Not for boiling points at pressures other than 1 atm, such as cooking at altitude.

Common mistakes

  • Using molarity in mol/L instead of molality in mol per kg of solvent.
  • Dividing by the mass of the whole solution rather than the solvent alone.
  • Leaving i at 1 for salts like NaCl or CaCl2 that dissociate into ions.
  • Adding the elevation in °F terms; ΔTb is in °C, which equals kelvin.

Frequently asked questions

Why does dissolving salt raise the boiling point of water?

Dissolved particles reduce the fraction of solvent molecules at the surface, which lowers the vapour pressure. The solution needs a higher temperature before its vapour pressure matches atmospheric pressure, so it boils later.

How much does salt in pasta water actually raise its boiling point?

Very little. A typical 10 g of salt per litre is about 0.17 mol/kg; with i = 2 the rise is 2 × 0.512 × 0.17, roughly 0.18 °C. The salt is there for taste, not temperature.

What value of the van 't Hoff factor should I use?

Use 1 for sugars, urea and other molecules that do not ionise. For strong electrolytes use the ions per formula unit: 2 for NaCl or KBr, 3 for CaCl2 or Na2SO4. Measured values are often a little lower because ions pair up in solution.

Why is molality used instead of molarity?

Molality is moles per kilogram of solvent, which does not change with temperature. Molarity depends on solution volume, and volume expands as the liquid is heated toward boiling.

Can this find the molar mass of an unknown solute?

Not directly, but you can work backwards: measure ΔTb, divide by i × Kb to get molality, then divide the grams of solute per kg of solvent by that molality. Trying molar masses in the Masses mode until ΔTb matches also works.

Is the boiling point elevation the same in kelvin and Celsius?

Yes. A temperature difference of 1 °C equals 1 K, so ΔTb has the same number in both. Only Fahrenheit differs, by a factor of 1.8, which is why the °F result is converted from the final boiling point.

Cite this tool

Using this tool in an assignment, article or report? Copy a citation, or a link for your website.

APA

Tools Directory Editorial Team. (2026, October 27). Boiling Point Elevation. Tools Directory. Retrieved October 27, 2026, from https://tools.milunir.com/chemistry/gas-laws-and-thermodynamics/boiling-point-elevation/

MLA

Tools Directory Editorial Team. “Boiling Point Elevation.” Tools Directory, 27 Oct. 2026, https://tools.milunir.com/chemistry/gas-laws-and-thermodynamics/boiling-point-elevation/. Accessed 27 Oct. 2026.

Chicago

Tools Directory Editorial Team. “Boiling Point Elevation.” Tools Directory. Last modified October 27, 2026. Accessed October 27, 2026. https://tools.milunir.com/chemistry/gas-laws-and-thermodynamics/boiling-point-elevation/.

Link<a href="https://tools.milunir.com/chemistry/gas-laws-and-thermodynamics/boiling-point-elevation/">Boiling Point Elevation – Tools Directory</a>