Molality uses solvent mass, not solution volume
That definition is why molality stays useful when temperature changes the solution volume but not the amount of solvent present.
Molality Calculator
Inputs
Lowercase m means molality, not molarity.
Use solvent mass only, not total solution mass.
Enter solute mass, molar mass, and solvent mass to calculate molality using m = n / kg solvent and n = mass / molar mass.
Results
Molality result
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Molality in mmol/kg: -
Equivalent label: -
Material amounts
Solute moles: -
Solute mass: -
Solvent mass: -
Normalized inputs
Molar mass: -
Mass fraction basis: -
Temperature note: -
Interpretation
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Formula summary
| Quantity | Formula | Meaning |
|---|---|---|
| Molality | m = n / kg solvent | Moles of solute per kilogram of solvent. |
| Solute moles | n = mass / MW | Moles from solute mass and molar mass. |
| Target solute mass | mass = m x kg solvent x MW | Mass of solute needed for a target molality. |
| Target solvent mass | kg solvent = n / m | Solvent mass required to reach a target molality. |
| Target moles | n = m x kg solvent | Solute amount needed for a known solvent mass. |
Molality uses kilograms of solvent, not liters of solution. That is why it stays useful when solution volume changes with temperature.
Worked examples
| Case | Inputs | Result |
|---|---|---|
| Calculate molality | 58.44 g NaCl, MW 58.44 g/mol, 0.500 kg water | 2.000 mol/kg. |
| Find solute mass | 1.500 mol/kg, 0.800 kg solvent, MW 180.156 g/mol glucose | 216.187 g solute. |
| Find solvent mass | 10.00 g solute, MW 60.052 g/mol urea, target 0.500 mol/kg | 0.333 kg solvent. |
For dilute aqueous solutions, molality and molarity can be numerically close, but they are not interchangeable because one uses solvent mass and the other uses solution volume.
How this molality calculator works
The page converts any entered solute mass to moles using the molar mass, then divides by kilograms of solvent. When you solve in reverse, it rearranges the same relationship to recover the missing solute mass, solvent mass, or moles.
Molality is often used in colligative-property work and other temperature-sensitive solution problems because the solvent mass does not depend on thermal expansion the way a solution volume does. That makes molality especially convenient for freezing-point depression, boiling-point elevation, and careful solution-composition comparisons.
m = n / kg solventn = mass / MWmass = m x kg x MWkg = n / m
Input guidance
- Solvent mass only: enter the mass of the solvent, not the total solution mass.
- Molar mass required: if you want the calculator to move between grams and moles, provide a positive molar mass.
- Target mode: for target solute mass, target solvent mass, and target moles, enter the desired molality first.
- Unit awareness: the result is normalized to mol/kg even if you enter grams or milligrams.
5 Fun Facts about Molality
Colligative properties love molality
Freezing-point depression and boiling-point elevation formulas are commonly written in terms of molality because they depend on particle amount relative to solvent mass.
Dilute solutions can hide the difference
For very dilute water-based mixtures, molality and molarity can look numerically similar, but they are still different quantities with different units.
Solvent mass means solvent only
Students often enter total solution mass by mistake. In molality, only the solvent mass belongs in the denominator.
Sea-water style comparisons benefit from mass basis
When density changes matter, a mass-based concentration like molality can be easier to compare across temperatures than a volume-based concentration.
FAQ
Is molality the same as molarity?
No. Molarity is moles per liter of solution, while molality is moles per kilogram of solvent.
Why does molality stay more stable with temperature?
Because mass is not affected the same way volume is. A warmed solution may expand, changing liters of solution, but the solvent mass does not change from that expansion alone.
Can I use this for lab prep directly?
It is useful for planning and study, but it does not correct for purity, hydrates, density, or solvent losses. Use your lab method where those matter.
Does this calculator upload my data?
No. The page runs entirely in your browser.
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Limits and lab note
This page performs concentration arithmetic only. It does not derive molar mass from a formula, estimate density, convert molality to molarity, or correct for purity, hydration state, evaporation, or non-ideal solution behavior. Verify your method before using the result in lab work.