What this calculator does
Molality is moles of solute per kilogram of solvent. Ten grams of sodium chloride in 1 kg of water gives 0.1711 mol/kg.
The difference from molarity is the denominator: kilograms of solvent rather than litres of solution. That makes molality independent of temperature, since mass does not expand when heated while volume does. It is the concentration measure used for colligative properties precisely because those calculations must hold across a range of temperatures.
The formula
The solute mass is divided by its molar mass to give moles, which is divided by the solvent mass in kilograms. Note that only the solvent is counted, not the total solution, which is the opposite convention to mass percent and a frequent source of confusion.
| Term | Meaning |
|---|---|
| Molality (b or m) | Moles of solute per kilogram of solvent. |
| Molarity (M) | Moles per litre of solution, which changes with temperature. |
| Colligative property | One depending on particle concentration, for which molality is the correct measure. |
| Solvent mass | The denominator here, excluding the solute. |
The inputs explained
| Field | What to enter |
|---|---|
| Mass of solute (g) | Mass of solute in grams. |
| Molar mass of solute (g/mol) | Molar mass of the solute in g/mol. |
| Mass of solvent (kg) | Mass of solvent in kilograms, not the total solution mass. |
When to use it
Colligative property calculations
Freezing point depression and boiling point elevation are both proportional to molality, not molarity.
Working across temperatures
A solution prepared and used at different temperatures keeps its molality but not its molarity.
Very concentrated solutions
Where the solute contributes substantially to the volume, molality remains well defined while molarity becomes awkward.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How does the solvent mass change molality?
The same solute in different quantities of solvent.
| Mass of solvent | Molality | Moles of solute | Concentration |
|---|---|---|---|
| 0.25 kg | 0.6845 mol/kg | 0.1711 mol | 684.5 mmol/kg |
| 0.5 kg | 0.3422 mol/kg | 0.1711 mol | 342.2 mmol/kg |
| 1 kg | 0.1711 mol/kg | 0.1711 mol | 171.1 mmol/kg |
| 2 kg | 0.0856 mol/kg | 0.1711 mol | 85.6 mmol/kg |
Questions
What is the difference between molality and molarity?
Molality is per kilogram of solvent, molarity is per litre of solution. Molality counts only the solvent and uses mass, so it does not change with temperature. Molarity includes the solute in the volume and does change, because liquids expand when heated.
When should I use molality?
For colligative properties: freezing point depression, boiling point elevation and vapour pressure lowering are all proportional to molality. Also whenever a solution will be used across a range of temperatures, where a molarity figure would drift.
Are molality and molarity ever the same?
Approximately, for dilute aqueous solutions, because a litre of dilute solution weighs close to a kilogram and contains close to a kilogram of water. The approximation degrades as concentration rises and fails entirely for non-aqueous solvents with different densities.
Do I use the solvent mass or the solution mass?
The solvent only. This is the opposite of mass percent, which uses the total. Ten grams of solute in 1 kg of water is 0.1711 molal even though the solution weighs 1.01 kg. Mixing the two conventions is the usual error.
For the volume-based measure, see the molarity calculator. For a colligative property it feeds, see the freezing point depression calculator.