Boiling Point Calculator
Calculate the boiling point elevation of a solution using the colligative property formula ΔTb = Kb × m × i. Enter the solvent, molality of solute, and van't Hoff factor to determine how much the boiling point increases when a solute is dissolved. Essential for physical chemistry, food science, and chemical engineering. See also our Freezing Point Depression Calculator and Osmotic Pressure Calculator.
mol solute / kg solvent
NaCl=2, CaCl₂=3, glucose=1
How to Calculate Boiling Point Elevation
Boiling point elevation is a colligative property — it depends only on the number of solute particles in solution, not on their chemical identity. When a non-volatile solute is dissolved in a solvent, the boiling point of the solution is always higher than that of the pure solvent. This occurs because solute particles lower the vapor pressure of the solvent, requiring a higher temperature to reach the atmospheric pressure needed for boiling.
The magnitude of boiling point elevation is directly proportional to the molal concentration of solute particles. For electrolytes that dissociate in solution (like NaCl → Na⁺ + Cl⁻), the van't Hoff factor (i) accounts for the increased number of particles. Ideal behavior assumes complete dissociation, though in practice, ion pairing reduces the effective i value slightly.
- Identify the solvent and look up its ebullioscopic constant (Kb)
- Calculate the molality of the solution (moles solute / kg solvent)
- Determine the van't Hoff factor (i) based on dissociation
- Calculate ΔTb = Kb × m × i
- Add ΔTb to the normal boiling point to get the new boiling point
Formula
Boiling Point Elevation:
ΔTb = Kb × m × i
Where:
ΔTb = boiling point elevation (°C)
Kb = ebullioscopic constant (°C·kg/mol)
m = molality (mol solute / kg solvent)
i = van't Hoff factor (number of particles per formula unit)
New Boiling Point:
T_new = T_normal + ΔTb
Van't Hoff Factor Examples:
Glucose (C₆H₁₂O₆): i = 1 (non-electrolyte)
NaCl: i = 2 (Na⁺ + Cl⁻)
CaCl₂: i = 3 (Ca²⁺ + 2Cl⁻)
Al₂(SO₄)₃: i = 5 (2Al³⁺ + 3SO₄²⁻)
Example Calculation
Problem: Calculate the boiling point of a solution made by dissolving 58.44 g of NaCl in 1 kg of water.
Given: Kb(water) = 0.512 °C/m, MW(NaCl) = 58.44 g/mol, i = 2
Solution:
Molality = 58.44 g ÷ 58.44 g/mol ÷ 1 kg = 1.00 m
ΔTb = 0.512 × 1.00 × 2 = 1.024 °C
New BP = 100.0 + 1.024 = 101.024 °C
Problem 2: 90 g of glucose (MW=180) in 500 g of water.
Molality = (90/180) / 0.5 = 1.0 m, i = 1
ΔTb = 0.512 × 1.0 × 1 = 0.512 °C → BP = 100.512 °C
Ebullioscopic Constants Reference Table
| Solvent | Kb (°C·kg/mol) | Normal BP (°C) | Kf (°C·kg/mol) |
|---|---|---|---|
| Water | 0.512 | 100.0 | 1.86 |
| Benzene | 2.53 | 80.1 | 5.12 |
| Ethanol | 1.22 | 78.4 | 1.99 |
| Chloroform | 3.63 | 61.2 | 4.68 |
| Acetic acid | 3.07 | 118.1 | 3.90 |
| Carbon tetrachloride | 5.03 | 76.7 | 29.8 |
| Diethyl ether | 2.02 | 34.6 | 1.79 |
| Acetone | 1.71 | 56.1 | 2.40 |
| Cyclohexane | 2.79 | 80.7 | 20.0 |
| Camphor | 5.95 | 204.0 | 37.7 |
Understanding Colligative Properties
Colligative properties are physical properties of solutions that depend on the number of dissolved particles rather than their chemical nature. The four main colligative properties are boiling point elevation, freezing point depression, vapor pressure lowering (Raoult's law), and osmotic pressure. All four are related through thermodynamic principles and share the common feature of depending on molal concentration and the van't Hoff factor.
The molecular basis for boiling point elevation lies in entropy. When a solute is added to a solvent, the entropy of the liquid phase increases (more disorder), making the liquid phase relatively more stable. This means more energy (higher temperature) is needed to overcome the liquid-gas transition. The effect is proportional to the number of solute particles because each particle contributes independently to the entropy increase.
Boiling point elevation has practical applications in cooking (adding salt to water raises the boiling point slightly), automotive coolants (ethylene glycol raises the boiling point of radiator fluid), and industrial processes. It is also used analytically to determine molar masses of unknown solutes — by measuring the boiling point elevation of a known mass of solute in a known mass of solvent, the molar mass can be calculated.
Frequently Asked Questions
What is boiling point elevation?
Boiling point elevation is the increase in boiling point that occurs when a non-volatile solute is dissolved in a solvent. It is a colligative property that depends on the number of solute particles, not their identity. The elevation is calculated as ΔTb = Kb × m × i.
What is the van't Hoff factor?
The van't Hoff factor (i) represents the number of particles a solute produces when dissolved. For non-electrolytes like glucose, i = 1. For NaCl, i = 2 (one Na⁺ and one Cl⁻). For CaCl₂, i = 3. In practice, i may be slightly less than the theoretical value due to ion pairing.
Why does adding salt raise the boiling point of water?
Salt (NaCl) dissociates into Na⁺ and Cl⁻ ions, which lower the vapor pressure of water by occupying surface positions and reducing the number of water molecules that can escape to the gas phase. A higher temperature is then needed to raise the vapor pressure to atmospheric pressure for boiling.
What is the ebullioscopic constant?
The ebullioscopic constant (Kb) is a property of the solvent that relates molal concentration to boiling point elevation. It depends on the solvent's molar mass, boiling point, and enthalpy of vaporization: Kb = RT²M / (1000 × ΔHvap). Water has Kb = 0.512 °C·kg/mol.
Does boiling point elevation work for volatile solutes?
The simple formula ΔTb = Kb × m × i applies only to non-volatile solutes. If the solute is volatile (has significant vapor pressure), both components contribute to the total vapor pressure, and the system follows Raoult's law for ideal solutions or requires activity coefficients for non-ideal behavior.
How is boiling point elevation used to find molar mass?
Dissolve a known mass of solute in a known mass of solvent, measure ΔTb, then: MW = (Kb × mass_solute × i) / (ΔTb × mass_solvent_kg). This ebullioscopic method works best for non-volatile, non-electrolyte solutes where i = 1.