Molality Calculator
Molality from mass or moles of solute and the solvent mass — the concentration unit that does not change with temperature.
Molality is moles of solute per kilogram of *solvent* — not of solution, which is the difference from molarity.
How the molality calculator works
Molality is moles of solute per kilogram of *solvent* — not of solution, which is the difference from molarity.
That distinction matters because volume expands with temperature and mass does not. A 1 M solution is no longer 1 M when heated; a 1 m solution stays 1 m. Colligative properties are defined in molality for exactly this reason.
Formula: m = moles solute / kg solvent
Worked examples
| Inputs | Molality | Note |
|---|---|---|
| 58.44 g of NaCl in 1 kg of water | 1 mol/kg | 1 molal, 2 osmolal |
| Half the solvent | 2 mol/kg | 2 molal |
| Sugar instead | 1 mol/kg | 1 molal but only 1 osmolal |
FAQFrequently asked questions
What is the difference between molarity and molality?
Molarity is per litre of solution; molality is per kilogram of solvent. They are close for dilute aqueous solutions and diverge as concentration rises.
Why use molality?
Because mass does not change with temperature and volume does. A molal solution stays molal when heated.
What is osmolality?
Molality multiplied by the number of particles each formula unit produces. Sodium chloride at 1 molal is 2 osmolal.
Which do colligative properties use?
Molality. That is why the boiling and freezing constants are quoted per molal.
When are they nearly equal?
For dilute aqueous solutions below about 0.1 M, where a litre of solution is close to a kilogram of water.
Where these figures come from
- IUPAC — Standard atomic weights (2021 conventional values) — the molar-mass table
- NIST — CODATA 2018 fundamental physical constants — Avogadro constant, gas constant, speed of light
- NIST Chemistry WebBook — thermochemical data
- CSIRO — Australia's national science agency
Last checked: September 2026. Atomic masses are the IUPAC conventional values; constants are CODATA 2018; equations are the standard textbook forms.