Last updated: July 3, 2026
Boiling Point Elevation Calculator
Creators
Dharmendra SinghReviewers
Creators
Dharmendra SinghReviewers
Quick Answer
The boiling point elevation calculator applies ΔTb = iKb m. Enter van 't Hoff factor, solvent ebullioscopic constant, and molality to get the boiling point increase and new boiling point; or enter ΔTb to solve for molality or i.
Boiling point elevation equals the van 't Hoff factor times the ebullioscopic constant times molality. For water, one molal nonelectrolyte raises the boiling point by zero point five one two degrees Celsius.
Key Takeaways
- Boiling point elevation follows ΔTb = i × Kb × m.
- Water has Kb = 0.512 °C·kg/mol and normally boils at 100 °C at 1 atm.
- The new solution boiling point is the pure-solvent boiling point plus ΔTb.
- Use molality in mol/kg because colligative constants are tabulated per solvent mass.
- Electrolytes need an effective van 't Hoff factor, not always the ideal integer value.
Creators
Dharmendra SinghReviewers
Creators
Dharmendra SinghReviewers
Formula
ΔTb = i × Kb × m; Tb(solution) = Tb° + ΔTb
Where:
- ΔTb=Boiling point elevation(°C)
- i=van 't Hoff factor(dimensionless)
- Kb=Ebullioscopic constant(°C·kg/mol)
- m=Molality(mol/kg)
- Tb°=Normal boiling point of pure solvent(°C)
Worked Examples
1 molal nonelectrolyte in water
A molecular solute with i = 1 is dissolved at 1 mol/kg in water.
- 1Use ΔTb = i × Kb × m.
- 2Substitute i = 1, Kb = 0.512 °C·kg/mol, and m = 1 mol/kg.
- 3ΔTb = 1 × 0.512 × 1 = 0.512 °C, so Tb = 100 + 0.512 = 100.512 °C.
Ideal sodium chloride solution
NaCl is approximated as i = 2 at 0.5 mol/kg in water.
- 1Use ΔTb = iKb m.
- 2Substitute i = 2, Kb = 0.512, and m = 0.5.
- 3ΔTb = 2 × 0.512 × 0.5 = 0.512 °C.
2.5 molal nonelectrolyte
A concentrated nonelectrolyte in water raises the boiling point more strongly.
- 1Multiply the particle factor, solvent constant, and molality.
- 2ΔTb = 1 × 0.512 × 2.5 = 1.28 °C.
- 3The new boiling point is 100 + 1.28 = 101.28 °C.
Introduction
Boiling point elevation is a colligative property: dissolved particles lower the solvent vapour pressure, so the solution must be heated to a higher temperature before it boils. This calculator uses ΔTb = iKb m and adds the result to the pure-solvent boiling point. Pair it with the molality calculator when you need m from mass data, or compare the particle-count logic with the osmotic pressure calculator. Background definitions are summarized by IUPAC colligative terminology and by LibreTexts colligative properties.
Boiling point elevation formula
The equation is ΔTb = i × Kb × m. ΔTb is the temperature increase in °C, i is the van 't Hoff factor, Kb is the ebullioscopic constant of the solvent, and m is molality in mol/kg. The solution boiling point is Tb° + ΔTb.
Water Kb = 0.512 °C·kg/mol.
Use molality, not molarity, in the colligative equation.
The same numerical size applies in kelvin or degrees Celsius for a temperature difference.
For the final boiling point, add ΔTb to the pure solvent boiling point.
How to calculate ΔTb
Choose an effective particle factor, enter the solvent Kb, and provide molality. For an aqueous nonelectrolyte, i = 1 and Kb = 0.512. For an ideal NaCl estimate, i = 2. The calculator can also solve backward: enter ΔTb and i to find molality, or enter ΔTb and molality to estimate i.
The default normal boiling point is 100 °C for water at 1 atm.
Choosing the van 't Hoff factor
The van 't Hoff factor counts effective dissolved particles per formula unit. Glucose and sucrose remain molecules, so i is about 1. NaCl ideally gives 2 particles, CaCl₂ gives 3, but real electrolyte solutions can have lower effective values because of ion pairing and activity effects.
| Solute | Ideal i | Comment |
|---|---|---|
| Sucrose | 1 | Nonelectrolyte |
| NaCl | 2 | Na⁺ + Cl⁻ |
| CaCl₂ | 3 | Ca²⁺ + 2Cl⁻ |
| AlCl₃ | 4 | Al³⁺ + 3Cl⁻ ideal limit |
Why the equation uses molality
Colligative boiling point elevation is tabulated against molality because solvent mass does not expand with temperature. Molarity can change as a solution warms. If you measured grams of solute and solvent, first calculate molality with the molarity calculator only for volume-based concentration checks and use the mole fraction calculator for composition ratios.
For colligative properties, keep solvent mass separate from total solution mass.
Assumptions and limitations
The simple formula is most reliable for dilute, nearly ideal solutions with nonvolatile solutes. It does not handle volatile solutes, high ionic strength, activity-coefficient corrections, azeotropes, pressure changes, or decomposition before boiling. Solvent constants are empirical and depend on the pure solvent.
Use an experimentally appropriate Kb for non-water solvents.
Use effective rather than ideal i for concentrated electrolytes.
Account for pressure separately; altitude changes pure-solvent boiling point.
Report the solvent and pressure with precise results.
Laboratory applications
Boiling point elevation appears in molecular mass determination, antifreeze and heat-transfer fluids, food and sugar solutions, and physical chemistry demonstrations. For boiling at reduced pressure or high elevation, combine the new normal boiling point estimate with a pressure-specific model such as the boiling point at altitude calculator. For unit conventions, consult the IUPAC Green Book.
Quick Reference Card
Boiling Point Elevation — Quick Reference
Quick reference • Boiling Point Elevation Calculator
ΔTb = i × Kb × m; Tb = Tb° + ΔTbValid range: Best for dilute, ideal solutions of nonvolatile solutes below solubility and decomposition limits.
Common Values
⚠ Watch Out
- •Do not enter molarity where molality is required.
- •Do not use the water Kb for another solvent.
- •Ideal i values can overestimate concentrated electrolyte solutions.
- •Normal boiling point changes with pressure, so specify pressure for precision.
- •The formula assumes a nonvolatile solute and a nearly ideal dilute solution.
Pro Tips
- →Use i = 1 for glucose, sucrose, and other nonelectrolytes.
- →For quick aqueous estimates, multiply 0.512 by i and molality.
- →Calculate molality from weighed solute and solvent before using the colligative equation.
- →Keep extra digits through intermediate steps and round the final displayed temperature.
- →If boiling occurs at altitude, adjust Tb° before adding ΔTb.
FAQs
What is boiling point elevation?
It is the increase in a solvent's boiling point caused by dissolved nonvolatile solute particles.
What equation does this calculator use?
It uses ΔTb = i × Kb × m and then adds ΔTb to the normal boiling point to estimate the solution boiling point.
What is Kb for water?
The ebullioscopic constant of water is commonly tabulated as 0.512 °C·kg/mol.
Should I enter molarity or molality?
Enter molality in mol/kg. Colligative boiling point elevation uses solvent mass, not solution volume.
Can the calculator solve for molality?
Yes. Enter a known ΔTb, van 't Hoff factor, and Kb; the calculator rearranges to m = ΔTb/(iKb).
Why is the real value sometimes lower than the ideal result?
Concentrated electrolytes can show ion pairing and nonideal activity, which lower the effective particle count compared with an ideal integer i.