Molality calculator
Enter the solute formula, its mass and the solvent mass in grams. Molality does not change with temperature, so it is used for freezing point and boiling point problems.
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How to use it
- Enter solute formula, solute mass and solvent mass.
- The result appears straight away and updates as you edit.
- Copy the result, or copy a link that keeps the same values to share it.
How it is calculated
Molality b (mol/kg) = (solute mass ÷ molar mass) ÷ solvent mass in kilograms.
Example
Inputs: Solute formula = C6H12O6, Solute mass = 18.0156 g, Solvent mass = 250 g. Result: Molality: 0.4 mol/kg; Moles of solute: 0.1 mol; Molar mass: 180.156 g/mol.
Limits and caveats
- Does not count ions from dissociation; for colligative properties multiply by the van 't Hoff factor.
- Use the mass of the solvent alone, not of the whole solution.
Questions about this tool
When should I use molality instead of molarity?
When temperature changes and in colligative property work, since mass does not change with temperature the way volume does.
Are molality and molarity equal in water?
They are close in dilute aqueous solutions and drift apart as concentration rises.
What is the unit of molality?
Moles per kilogram of solvent, sometimes written as a lowercase m.