Molality Calculator
Enter any two of molality, moles of solute and kilograms of solvent to solve the third. A live 3D beaker and charts show how molality differs from molarity.
Reviewed by the ToolNestr Editorial Team — July 2026
Enter any two values — the third solves instantly.
Live 3D molality
Molality divides by the solvent mass. Add solvent (raise the level) and the same solute spreads thinner; add solute and the particle density climbs. Drag to orbit.
Molality graphs
How molality is calculated
The core definition in one line: molality is moles of solute divided by kilograms of solvent — m = n / kg. The key word is solvent, not solution.
It rearranges the usual three ways:
m = n / kg
molality from moles & solvent mass
n = m × kg
moles of solute needed
kg = n / m
solvent mass required
Because it is built on mass, molality never changes with temperature — a huge advantage over molarity, whose volume-based denominator drifts as the solution expands or contracts. That is why the colligative-property equations ΔTf = Kf·m and ΔTb = Kb·m are written in molality.
Worked example 1 — molality from mass
Given: dissolve 5.85 g of NaCl (molar mass 58.44 g/mol) in 0.500 kg of water. Find the molality.
Worked example 2 — antifreeze & freezing point
Given: a 2.0 mol/kg solution of a non-ionising solute in water (Kf = 1.86 °C·kg/mol). How far does the freezing point drop?
This is exactly why molality — not molarity — is the right unit here: the solution is being cooled, and only molality stays constant as it does.
Two ideas that trip students up
1. Per kg of solvent, not solution
The denominator is the water alone (the blue liquid mass), measured before the solute is added — not the total mixed solution. That's the difference from molarity.
2. Heat it — molality holds
Warm the beaker and the liquid expands (its volume grows, so molarity falls), but the mass — and therefore the molality — is unchanged. Mass doesn't care about temperature.
Molality vs molarity at a glance
| Property | Molality (m) | Molarity (M) |
|---|---|---|
| Divides by | kg of solvent | L of solution |
| Unit | mol/kg | mol/L |
| Temperature-dependent? | No ★ | Yes |
| Best for | colligative / thermo | volumetric work |
★ Mass is temperature-independent, so molality is too — the reason freezing/boiling-point formulas use it.
Where molality actually matters
❄️ Antifreeze & de-icing
Freezing-point depression, ΔTf = Kf·m, is set by molality. Engine coolant and road salt are dosed to hit a target freeze point — a calculation that must survive being cooled, so molarity won't do.
🌡️ Boiling-point elevation
Adding solute raises the boiling point by ΔTb = Kb·m. Cooking pasta water and industrial evaporators both feel this molality-driven shift.
🔬 Precise thermodynamics
Any measurement across a temperature range — activity coefficients, calorimetry, high-temperature reactions — uses molality so the concentration figure doesn't secretly change with the thermometer.
Common misconceptions
"Molality divides by the solution mass."
It divides by the solvent mass only. Weigh the water (or other solvent) before dissolving — not the total mixture.
"Molality and molarity are the same for water."
They're close for dilute aqueous solutions (1 L water ≈ 1 kg) but diverge as concentration rises, and they respond differently to temperature. They are not interchangeable in general.
"Use litres for the denominator."
Molality uses kilograms of solvent, a mass. If you only have a volume, convert with the solvent density first (1 L water ≈ 1 kg).
"Heating a solution changes its molality."
It doesn't. Heating changes volume (and hence molarity), but the masses of solute and solvent are unchanged, so molality is constant — its whole point.
Formula sources & further reading
Molality and the colligative-property relations are standard general chemistry, traceable to:
- • OpenStax, Chemistry 2e — §11.4 "Colligative Properties" and §3.4 (molality). openstax.org
- • Brown, LeMay & Bursten, Chemistry: The Central Science — Chapter 13, Properties of Solutions.
- • Zumdahl & Zumdahl, Chemistry — Chapter 11, Properties of Solutions.
Water: Kf = 1.86 °C·kg/mol, Kb = 0.512 °C·kg/mol. Results are rounded for display.
How to use this calculator
Enter two values
Fill any two of molality, moles of solute and kilograms of solvent.
Read the third
The missing quantity solves instantly from m = mol/kg.
Explore in 3D
Use the sliders to see how the solvent mass drives the concentration.
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Frequently asked questions
What is molality?
Molality (m) is the moles of solute per kilogram of solvent: m = mol / kg. Its unit is mol/kg, sometimes written m or molal. Note it uses the mass of the solvent, not the volume of the solution.
How is molality different from molarity?
Molarity is moles per litre of solution and changes slightly with temperature because volume expands or contracts. Molality is moles per kilogram of solvent and is temperature-independent, since mass never changes.
When should I use molality instead of molarity?
Use molality for temperature-dependent work: boiling-point elevation, freezing-point depression, and any careful thermodynamics where the solution will be heated or cooled. Molarity is fine for room-temperature volumetric work.
How do I calculate molality from mass?
Convert the solute mass to moles (mass ÷ molar mass), then divide by the solvent mass in kilograms. For 5.85 g NaCl (0.1 mol) in 0.5 kg water: m = 0.1 / 0.5 = 0.2 mol/kg.
Does molality use the solvent or the solution mass?
The solvent only. Molality divides by the kilograms of solvent (e.g. the water), not the total mass of the solution. This is the single most common molality mistake.
Why is molality used for freezing-point depression?
The colligative equations ΔTf = Kf·m and ΔTb = Kb·m are written in molality precisely because they must hold as temperature changes — and only molality stays constant when the solution is cooled or heated.