Skip to content
chemistrycalc
✓ Always ad-freeAbout us

Molality Calculator

Calculate moles of solute per kilogram of solvent, using either solute moles or mass.

By ChemistryCalc•Updated October 28, 2026•References
Your calculationLive results · change any value

Results update as you type. Choose units beside a measurement; changing units converts the current value.

On this pageCalculatorHow to useFormula & methodWorked exampleThe denominator is solvent massWhy molality is usefulFrequently asked questionsRelated calculatorsSources

01 Set up your calculation

  1. Choose whether the solute is specified by moles or by mass.
  2. Enter the solvent mass, excluding the dissolved solute.
  3. Read the molality in moles per kilogram of solvent.

02 The relationship

Molality b = moles of solute ÷ kilograms of solvent; solute moles = solute mass ÷ molar mass.

For the assumptions behind these results, read the explanation below. You can also explore Molarity and Mole.

WORKED EXAMPLE

Follow the numbers

Dissolving 0.5 mol of solute in 1 kg of solvent gives 0.5 mol/kg. The same amount in 0.25 kg of solvent gives 2 mol/kg.

03 The denominator is solvent mass

Molality uses the material doing the dissolving, not the complete solution. For 10 g of solute added to 90 g of solvent, the denominator is 0.090 kg, even though the total solution weighs 100 g.

This distinction matters most for concentrated solutions. Replacing solvent mass with solution mass systematically lowers the calculated molality.

04 Why molality is useful

Mass does not expand with temperature the way a measured liquid volume does. Molality is therefore convenient for many calculations involving changes in freezing point or boiling point.

The result is the analytical molality of the entered solute. It does not automatically multiply for dissociation into ions. A colligative-property calculation may need a separate van ’t Hoff factor or a more complete solution model.

Questions from the notebook

Is molality the same as molarity?

No. Molality is per kilogram of solvent; molarity is per liter of solution.

Can I use milligrams for the solute?

Yes. Choose that mass unit and enter the molar mass consistently. The unit controls perform the conversion.

References & reading

Our explanations are written for ChemistryCalc. Read how we build and check our tools. These calculations are educational estimates where the underlying model requires assumptions.