Calculates molality — moles of solute per kilogram of solvent, a concentration unit that doesn’t change with temperature.
How it works
Moles of solute are divided by the solvent’s mass in kilograms.
What this does not include
This does not include dividing by solution volume instead — for that different concentration unit (molarity), use this site’s molarity calculator instead.
How to use this calculator
- Enter the moles of solute and the mass of solvent in kilograms.
A worked example
2 moles of solute dissolved in 0.5 kg of solvent: molality = 2 ÷ 0.5 = 4 mol/kg.
0.5 moles in 1 kg of solvent: molality = 0.5 mol/kg.
What the variables mean
| Variable | Meaning |
|---|---|
| Moles of solute | Amount of dissolved substance |
| Solvent mass (kg) | Mass of the solvent only, not the total solution |
Edge cases worth knowing
Molality uses solvent mass, not total solution mass or volume — the key distinction from molarity, which divides by solution volume instead. This makes molality unaffected by temperature-driven volume changes.
Zero solvent mass makes molality undefined — there’s no solvent to dissolve anything into, so the calculator returns no result.
Frequently asked questions
Why use molality instead of molarity?
Molality is based on mass, which doesn’t change with temperature — molarity is based on volume, which expands or contracts as temperature changes, making molality more stable for precise work like freezing point or boiling point calculations.
Does molality use solvent mass or total solution mass?
Specifically the solvent’s mass alone, not the combined solution — this is a key difference from mass percent, which uses the total solution mass.
Where is molality commonly used?
Colligative property calculations, like boiling point elevation and freezing point depression, rely on molality specifically because of its temperature independence.
