Molality Calculator
Calculate molality (moles of solute per kilogram of solvent), the temperature-independent measure of concentration.
How it works
b = moles of solute / kilograms of solvent
Molality states concentration as moles of solute per kilogram of solvent — note the solvent mass, not the solution volume. Because mass does not change with temperature, molality stays constant even when a solution is heated or cooled, which makes it the preferred unit for boiling-point and freezing-point calculations.
Worked example
Dissolving 0.5 mol of solute in 1 kg of water gives a molality of 0.5 ÷ 1 = 0.5 mol/kg.
Frequently asked questions
How is molality different from molarity?
Molality uses kilograms of solvent, molarity uses litres of solution. Molality is unaffected by temperature; molarity shifts slightly because volume expands when warmed.
When should I use molality?
Use it for colligative properties such as freezing-point depression and boiling-point elevation, and whenever temperature will vary during the experiment.
Is the mass the solvent or the whole solution?
Only the solvent — usually the water. The solute’s mass is not included in the denominator, unlike some other concentration measures.
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