From moles: 3 mol in 0.5 kg solvent
m = n / kg = 3 / 0.5 = 6 mol/kg
Molality Calculator
Enter any two values
Leave exactly one of moles, solvent mass, or molality blank.
Lowercase m means molality, not molarity.
Use solvent mass only, not total solution mass.
Use solute mass instead of moles
m = n / kg solvent. If the mass option is used, its calculated moles replace the moles field.
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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Calculation steps
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Formula checked: molality equals moles of solute per kilogram of solvent. Calculations run locally in your browser.
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.
Molality vs molarity
| Feature | Molality | Molarity |
|---|---|---|
| Definition | Moles per kilogram of solvent | Moles per litre of solution |
| Symbol | lowercase m | uppercase M |
| Unit | mol/kg | mol/L |
| Denominator | Solvent mass | Total solution volume |
| Temperature dependence | Essentially independent | Can change as volume changes |
| Common use | Colligative properties | Volumetric solution preparation |
Worked examples
From grams: 70.128 g NaCl in 1.5 kg water
n = 70.128 g / 58.44 g/mol = 1.2 mol
m = 1.2 / 1.5 = 0.8 mol/kg
Find moles for a 0.8 m solution
With 2.5 kg solvent: n = m × kg = 0.8 × 2.5 = 2 mol.
Find solvent mass
For 1.5 mol at 0.6 mol/kg: kg solvent = n / m = 1.5 / 0.6 = 2.5 kg.
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.
- Leave one blank: enter exactly two core values so the calculator can identify the unknown.
- Unit awareness: the result is normalized to mol/kg even if you enter grams or milligrams.
Common molality mistakes
- Solution mass vs solvent mass: subtract solute mass if only total solution mass is known.
- Grams vs kilograms: the denominator must be kilograms of solvent.
- Lowercase m vs uppercase M: m is molality; M is molarity.
- Purity and hydrates: correct the weighed amount for purity and use the molar mass of the actual hydrate.
- Temperature: prefer molality when temperature changes make solution volume unreliable.
Convert molality and molarity
These conversions require solution density and solute molar mass. Use density d in kg/L and molar mass W in kg/mol.
M = m × d / (1 + m × W); reverse: m = M / (d − M × W).
FAQ
How do I calculate molality?
Divide moles of solute by kilograms of solvent: m = n / kg solvent.
What is the molality of 3 moles in 0.5 kg solvent?
The molality is 3 / 0.5 = 6 mol/kg.
What unit is molality measured in?
Molality is measured in moles per kilogram of solvent, written mol/kg or mol kg^-1.
Do I use solvent mass or solution mass?
Use the mass of the solvent only, not the total mass of the solution.
How do I calculate molality from grams?
Divide solute mass in grams by molar mass in g/mol to get moles, then divide those moles by solvent mass in kilograms.
How do I convert molality to molarity?
Using density d in kg/L and molar mass W in kg/mol, calculate M = m × d / (1 + m × W).
Limits and lab note
This page performs concentration arithmetic only. It does not derive molar mass or density, or correct for purity, hydration state, evaporation, or non-ideal solution behavior. Verify your inputs and lab method.