Case 1
- Moles of solute (mol)
- 0.5mol
- Mass of solvent (kg)
- 2kg
0.2500mol/kg
Open with these valuesMolality Calculator
0.2500mol/kg
Result: 0.2500 mol/kgMolality is moles of solute per kilogram of solvent, in mol/kg — not per litre of solution, which is molarity. Weigh the solvent alone, before the solute goes in. Because a mass does not expand when heated, molality stays put where molarity drifts with temperature.
Worked examples
Case 2
- Moles of solute (mol)
- 1mol
- Mass of solvent (kg)
- 1kg
1.0000mol/kg
Open with these valuesCase 3
- Moles of solute (mol)
- 2mol
- Mass of solvent (kg)
- 0.5kg
4.0000mol/kg
Open with these valuesHow it's calculated
b = moles of solute ÷ kilograms of solvent
- StepTurn the solute's mass into moles: grams divided by its molar mass.
- StepWeigh the solvent alone in kilograms — not the finished solution.
- ResultDivide the moles by the kilograms; the answer is in mol/kg.
Reference table
| Moles, kg of solvent | What that is | Molality in mol/kg |
|---|---|---|
| 0.001, 0.25 | a trace in a small sample | 0.0040 |
| 0.5, 2 | half a mole in two kilograms | 0.2500 |
| 1, 1 | one mole in one kilogram — one molal | 1.0000 |
| 3, 1.5 | three moles in one and a half kilograms | 2.0000 |
| 2, 0.5 | two moles in half a kilogram | 4.0000 |
| 58.44, 1 | the mole count of a kilogram of table salt | 58.4400 |
Questions
How do I calculate molality?
Divide the moles of solute by the mass of the solvent in kilograms: b = moles ÷ kg. 0.5 mol dissolved in 2 kg of solvent gives 0.25 mol/kg.
What is the difference between molality and molarity?
Molality uses the mass of the solvent (mol/kg); molarity uses the volume of the whole solution (mol/L). Volume expands as the temperature rises and mass does not, so only molality stays constant.
Do I use the mass of the solvent or of the whole solution?
The solvent only — the liquid you dissolve into, not the finished mixture. This is the most common mistake, so weigh the solvent before the solute goes in.
Why doesn't molality change with temperature?
It rests on the mass of the solvent, and mass does not change when a sample is heated or cooled. That is why colligative properties such as freezing-point depression are written in molality.
Which units does molality use?
Moles per kilogram, written mol/kg and read as molal. If the solvent mass is in grams, divide it by 1000 first.
Sources and last check
Information, not professional advice.