Molality Calculator
Solve moles of solute, kilograms of solvent, or molality using m = n / kg solvent. Molality stays stable when temperature changes volume. Informational, not a lab protocol.
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How to use the molality calculator
- Enter exactly two of moles, solvent mass, and molality. Leave the unknown blank.
- Choose kilograms, grams, or milligrams for the solvent mass unit.
- Read molality in mol/kg, moles of solute, and solvent mass in kilograms.
- Use solvent mass only. Do not enter the mass of the whole solution unless that is what you measured as solvent.
Molality
Amount of solute per kilogram of solvent
| Symbol | m or mol/kg |
|---|---|
| Formula | m = n / kg solvent |
| Temperature | Independent of solution volume expansion |
| Solvent mass | Kilograms of solvent, not solution |
| Colligative use | Common in freezing-point and boiling-point problems |
| Versus molarity | Molarity uses liters of solution |
What molality measures
Molality is moles of solute divided by kilograms of solvent. A 2 mol/kg solution holds two moles of solute for every kilogram of solvent. The default 0.5 mol in 0.25 kg of solvent is 2 mol/kg.
Solvent mass versus solution mass
Weigh the solvent before or without counting dissolved solute mass as solvent. Solution mass includes solute. Mixing those two masses changes colligative calculations. This page always treats the mass field as solvent.
How this page solves m = n / kg
Provide any two of moles, solvent mass, and molality. Grams and milligrams convert to kilograms first. The page then fills the blank. Errors fire when zero or negative masses appear, or when the set of inputs is not exactly two known values.
Limits
Informational arithmetic only. Not a lab protocol, purity assay, or safety document. See the disclaimer.