Boiling Point Elevation Calculator
New boiling point of a solution from molality, the solvent constant Kb and the van't Hoff factor.
What it does
Add salt to water and it boils a little later. That is a colligative property: a change in a solution property that depends on how many particles are dissolved rather than on what those particles are. The same family of effects covers freezing point depression, vapour pressure lowering and osmotic pressure, and all of them come from the extra particles crowding the solvent.
The size of the effect depends on three things. The molality m is the number of moles of solute per kilogram of solvent. The constant Kb is a property of the solvent alone, and it says how resistant that solvent is to being pushed up to a boil. The van't Hoff factor i accounts for solutes that break apart, since one formula unit of sodium chloride gives two ions in solution while a sugar molecule stays whole.
The constants come from standard textbook tables, and this tool uses the values published in the OpenStax chemistry textbook for water, benzene, chloroform, glacial acetic acid and nitrobenzene. Pick the solvent from the list and the right Kb and normal boiling point are filled in for you.
The practical uses are ordinary. Antifreeze in a radiator works because it raises the boiling point and drops the freezing point at the same time. Sea water boils a little above 100 °C for the same reason, which is why pressure cookers work the other way, by raising the pressure so the water boils hotter.
Keep the ideal behaviour assumption in mind. The numbers are exact for dilute solutions that behave ideally, and they get less accurate as the concentration climbs or as the solute starts to interact with the solvent. For a lab calculation they are the numbers your textbook wants, and for a rough design estimate they are close enough to be useful.
How to use
- Choose the solvent. The constant Kb and the normal boiling point follow from it.
- Choose the unit system and enter the mass of solute, in grams or ounces.
- Enter the molar mass of the solute, which you find on the bottle or in the periodic table.
- Enter the mass of the solvent, not of the solution, in kilograms or pounds.
- Set the van't Hoff factor: 1 for a non-electrolyte such as sugar, close to 2 for a salt such as sodium chloride.
- Read the molality first, then the rise, then the new boiling point of the solution.
Formula
- m
- Molality of the solution, moles of solute per kilogram of solvent
- Kb
- Boiling point elevation constant of the solvent, in degrees Celsius per mol/kg
- i
- Van't Hoff factor, the number of particles each formula unit produces
- delta Tb
- How much the boiling point rises above the pure solvent, in degrees Celsius
- normal boiling point
- Boiling point of the pure solvent at 1 atm, in degrees Celsius
Molality uses the mass of the solvent alone, so weigh the solvent before you add anything to it. The equation is written for dilute solutions and assumes ideal behaviour. Measured van't Hoff factors sit a little below the ideal values, because ion pairs form in solution: the textbook table gives 1.9 rather than 2 for sodium chloride at 0.05 molal and 2.7 rather than 3 for magnesium chloride.
Worked example
Example: iodine dissolved in chloroform
92.1 g of iodine with a molar mass of 253.81 g/mol is dissolved in 800.0 g of chloroform. Iodine does not dissociate, so the van't Hoff factor is 1.
- Moles of iodine: 92.1 / 253.81 = 0.3629 mol.
- Solvent mass in kilograms: 800.0 g = 0.800 kg, so molality = 0.3629 / 0.800 = 0.454 mol/kg.
- Chloroform has Kb = 3.63, so the rise is 1 x 3.63 x 0.454 = 1.65 °C.
- Pure chloroform boils at 61.26 °C at 1 atm.
- The solution therefore boils at 61.26 + 1.65 = 62.91 °C.
- In Fahrenheit the same result reads as a rise of 2.97 °F, from 142.27 °F to 145.24 °F.
Reference table
| Solvent | Normal boiling point (°C) | Kb (°C per mol/kg) |
|---|---|---|
| Water | 100.0 | 0.512 |
| Benzene | 80.1 | 2.53 |
| Chloroform | 61.26 | 3.63 |
| Glacial acetic acid | 118.1 | 3.07 |
| Nitrobenzene | 210.9 | 5.24 |
Limits
- The result assumes ideal behaviour, which holds for dilute solutions and drifts as the concentration rises.
- Only the five solvents in the table are covered, and other solvents need their own Kb from a reference table.
- The solute must be a non-volatile substance, otherwise its own vapour pressure changes the answer.
- Atmospheric pressure is fixed at 1 atm, so the pure boiling point changes at altitude.
Common mistakes
- Using the mass of the solution instead of the mass of the solvent in the molality step.
- Leaving the van't Hoff factor at 1 for a salt that dissociates into two ions.
- Mixing up Kb with Kf, the freezing point constant, which has different values for every solvent.
- Dividing by the mass of the solvent in grams instead of kilograms and getting a result a thousand times too small.
- Expecting a large rise for salt water, when water's Kb is small and ordinary salt water boils barely above 100 °C.
Try it yourself
Quick checks. Answers sit tucked under each question.
What is the boiling point of a 0.10 molal solution of a non-volatile solute in benzene?
80.35 °C. The rise is 2.53 x 0.10 = 0.25 °C, and pure benzene boils at 80.1 °C, so the solution boils at 80.35 °C.
5.85 g of sodium chloride with a molar mass of 58.44 g/mol is dissolved in 0.100 kg of water with a van't Hoff factor of 1.9. What is the molality and the new boiling point?
Molality 1.001 mol/kg and a boiling point of 100.97 °C. Moles = 5.85 / 58.44 = 0.1001 mol, molality = 0.1001 / 0.100 = 1.001 mol/kg, and the rise is 1.9 x 0.512 x 1.001 = 0.97 °C above 100.00 °C.
Why does dissolving salt in water raise the boiling point at all?
The dissolved ions lower the vapour pressure of the water, so the solution needs more heat to reach the vapour pressure of the surrounding air. The extra heat shows up as a higher boiling point.
Questions
What is a colligative property?
A solution property that depends on the number of dissolved particles rather than on their identity. Boiling point elevation, freezing point depression, vapour pressure lowering and osmotic pressure are the four.
Why does the van't Hoff factor matter?
Because some solutes break apart. One formula unit of sodium chloride gives two ions, so the solution behaves like twice the molality. Sugar stays whole, so its factor is 1.
How much does salt raise the boiling point of water?
A little. Water has the lowest Kb in the table at 0.512, so even a concentrated salt solution climbs only a couple of degrees. Common salt has a factor near 1.9, so a 1 molal solution rises about 0.97 °C.
Why do pressure cookers use pressure instead of salt?
Water's Kb is small, so adding solute is a weak way to raise the boiling point. Raising the pressure instead lets the water boil at a much higher temperature, which is a far bigger change for the same pot.
Does the molar mass of the solute affect the answer?
It changes the molality, which is what drives the effect. For a fixed mass, a heavier solute gives fewer moles and therefore a smaller rise.
Can I use this for an electrolyte?
Yes, as long as you set the van't Hoff factor to the number of particles. Leave it at the ideal value for a full calculation or use a measured value from a reference table for a more realistic one.
Sources
- OpenStax Chemistry 2e, section 11.4 on colligative properties and Table 11.2
- Chemistry LibreTexts, OpenStax Chemistry 1e section 11.5: colligative properties