Boiling Point Elevation Calculator
Dissolving a non-volatile solute raises a solvent's boiling point in proportion to the number of particles released. Select a solvent, pick a solute preset or enter a custom van 't Hoff factor, and choose what you want to solve for: the elevation ΔTb, the solution boiling point, molality, the van 't Hoff factor, or the ebullioscopic constant Kb.
Formula
Worked example
For 58.44 g of NaCl (molar mass 58.44 g/mol) dissolved in 1000 g of water: molality = 1.00 mol/kg. With effective i = 1.9 and Kb = 0.512 °C·kg/mol: ΔTb = 1.9 × 0.512 × 1.00 = 0.973 °C. The solution boils near 100.97 °C. The ideal integer i = 2 gives 1.024 °C, slightly higher because of ion pairing.
What boiling point elevation is and why it happens
Boiling point elevation is the increase in a solvent's boiling temperature when a non-volatile solute dissolves in it. The underlying cause is vapour pressure lowering: dissolved particles occupy the surface of the liquid and reduce the fraction of solvent molecules that can escape into the vapour phase, so a higher temperature is needed before the vapour pressure matches the surrounding atmospheric pressure and boiling begins. Because the vapour pressure change depends only on the number of particles rather than their identity, boiling point elevation is called a colligative property. The quantitative relationship is ΔTb = i·Kb·m, where ΔTb is the temperature rise, i is the van 't Hoff factor, Kb is the ebullioscopic (molal boiling-point-elevation) constant of the solvent, and m is the molality of the solution.
The three variables and the four modes of this calculator
The van 't Hoff factor (i) counts particles per formula unit: 1 for molecular non-electrolytes such as glucose or urea, approximately 1.9 for NaCl (slightly below the ideal 2 due to ion pairing), about 2.7 for CaCl2, and about 3.4 for AlCl3. The ebullioscopic constant (Kb) depends on the solvent alone and is proportional to the square of its boiling point divided by its enthalpy of vaporisation times its molar mass. Molality (m) is moles of solute per kilogram of solvent, preferred over molarity because it is independent of temperature. This calculator lets you solve for any of the four quantities: the elevation ΔTb, molality, the van 't Hoff factor, or Kb. The reverse modes are valuable in laboratory settings where you measure the elevation and want to determine a concentration or identify a solvent.
Mass-based molality calculation
If you know the masses of solute and solvent rather than the molality directly, enable the "Calculate molality from masses" toggle. The calculator divides the solute mass in grams by its molar mass (g/mol) to get moles of solute, then divides by the solvent mass in kilograms: m = (solute mass / molar mass) / (solvent mass in kg). For example, 58.44 g of NaCl (molar mass 58.44 g/mol) in 1000 g of water gives exactly 1.00 mol/kg. This approach is convenient when you weigh reagents on a balance rather than measuring volume, and avoids the temperature dependence of volumetric measurements.
Ideal versus real solutions and when to apply corrections
The formula ΔTb = i·Kb·m is strictly valid only for ideal, infinitely dilute solutions. In real solutions, especially concentrated electrolyte solutions, ion pairing reduces the effective particle count below the nominal i. A 1 mol/kg NaCl solution has an effective i near 1.9 rather than 2.0, and a 1 mol/kg CaCl2 solution gives i near 2.7 rather than 3. At very high molalities the linear approximation breaks down entirely and the measured elevation can deviate substantially from the prediction. This calculator uses realistic effective i values for common electrolytes in the solute preset dropdown. For precise work at higher concentrations, consult experimental data or use the Pitzer activity coefficient model.
Practical applications
Boiling point elevation explains why salted water takes slightly longer to boil (the effect is real but small at culinary concentrations: 1 tsp salt in 1 L raises the boiling point by only about 0.1 °C). More practically, antifreeze solutions such as ethylene glycol raise the boiling point of engine coolant to reduce vapour lock at high engine temperatures, while also depressing the freezing point to prevent ice formation. In the laboratory, boiling point elevation combined with the known Kb of a solvent is used in ebullioscopy to measure the molar mass of an unknown solute, a technique particularly useful for polymers before modern mass spectrometry.
Ebullioscopic constants (Kb) and boiling points of common solvents
| Solvent | Kb (°C·kg/mol) | Normal boiling point (°C) | Kf (°C·kg/mol) |
|---|---|---|---|
| Water | 0.512 | 100 | 1.86 |
| Ethanol | 1.22 | 78.4 | 1.99 |
| Benzene | 2.53 | 80.1 | 5.12 |
| Chloroform | 3.63 | 61.2 | 4.68 |
| Acetic acid | 3.07 | 118.1 | 3.9 |
| Carbon tetrachloride | 4.95 | 76.7 | 29.8 |
| Phenol | 3.04 | 181.8 | 7.27 |
| Diethyl ether | 2.02 | 34.6 | 1.79 |
| Camphor | 5.95 | 204 | 39.7 |
Kb values in °C·kg/mol at the normal (1 atm) boiling point. Select any solvent from the dropdown above to auto-fill these values.
Frequently asked questions
What is the van 't Hoff factor and how do I choose it?
The van 't Hoff factor (i) is the number of particles a solute releases per formula unit when it dissolves. Use 1 for molecular non-electrolytes like sucrose or urea. For strong electrolytes the theoretical values are 2 for NaCl or KCl, 3 for CaCl2 or Na2SO4, and 4 for AlCl3, but in practice ion pairing in real solutions reduces these slightly (NaCl gives about 1.9 at 1 mol/kg). The solute preset in this calculator uses realistic effective values; choose "Custom" to enter your own.
How do I use this calculator to find the molar mass of a solute?
Switch to the "Solve for molality" mode and enter the measured boiling point elevation, the known Kb of your solvent, and an assumed i (use 1 for an unknown molecular solute). The calculator gives you the molality. Then rearrange m = (mass of solute / molar mass) / (mass of solvent in kg) to get molar mass = (mass of solute) / (m × mass of solvent in kg). For example, if 5.00 g of an unknown dissolved in 100 g of benzene raises the boiling point by 0.506 °C: m = 0.506 / (1 × 2.53) = 0.200 mol/kg, so molar mass = 5.00 / (0.200 × 0.100) = 250 g/mol.
Why does this calculator use molality rather than molarity?
Colligative property equations use molality (mol per kg of solvent) because it is defined by mass, which does not change with temperature. Molarity (mol per litre of solution) changes as the solution expands when heated toward its boiling point, which would introduce a small error in the calculation. Using molality also makes the derivation of the ΔTb formula thermodynamically exact for ideal dilute solutions.
How can I reverse-solve for Kb from an experimental measurement?
Select "Ebullioscopic constant (Kb)" from the "Solve for" dropdown. Enter the van 't Hoff factor for your solute, the molality of the solution, and the boiling point elevation you measured. The calculator rearranges ΔTb = i·Kb·m to Kb = ΔTb / (i·m) and shows you the result. This is how ebullioscopy was historically used to characterise solvents and measure polymer molecular weights.
Does boiling point elevation depend on the type of solute?
Not on its chemical identity, which is what makes it a colligative property. It depends only on how many dissolved particles are present, captured by the van 't Hoff factor and molality. A mole of dissolved sucrose molecules and a mole of dissolved Na+ ions each raise the boiling point by the same amount per particle. This is why equal-mass additions of sugar and table salt produce different elevations: NaCl dissociates into approximately twice as many particles as an equivalent mass of a molecular solute.
Can I enter temperatures in Fahrenheit or Kelvin?
Yes. Use the "Temperature unit for results" selector to switch all temperature outputs to °F or K. The internal calculation always uses °C, so the Kb and solvent boiling point values in the reference table (which are in °C) remain correct. Temperature differences (ΔTb) scale correctly between units: 1 °C elevation = 1.8 °F elevation = 1 K elevation.