Molality Calculator
Find molality (mol/kg) from moles of solute and kilograms of solvent — or solve for the moles or the solvent mass you need.
Quick answer: Molality (m) = moles of solute ÷ kilograms of solvent. Dissolving 0.50 mol of salt in 2.0 kg of water gives 0.50 ÷ 2.0 = 0.25 mol/kg. Unlike molarity, molality uses solvent mass, so it doesn't change with temperature.
Molality expresses concentration as moles of solute per kilogram of solvent. Because it's based on mass rather than volume, it doesn't change when the temperature changes — which is why it's the concentration unit used in colligative-property work like freezing-point depression and boiling-point elevation.
The formula
Molality (m) = moles of solute ÷ kilograms of solvent
Moles = m × kg · Solvent kg = moles ÷ m
Moles = m × kg · Solvent kg = moles ÷ m
Worked example
Dissolve 1.5 mol of glucose in 0.75 kg of water. Molality = 1.5 ÷ 0.75 = 2.0 mol/kg.
Frequently asked questions
What is molality?
Moles of solute per kilogram of solvent (mol/kg). It uses solvent mass, not solution volume.
How is molality different from molarity?
Molarity is moles per litre of solution; molality is moles per kilogram of solvent. Molality doesn't vary with temperature.
Do I use solvent mass or solution mass?
Solvent mass only — the mass of water or other solvent, not the total mass of the solution.
Why does molality matter for freezing point?
Colligative properties like freezing-point depression depend on molality, which stays constant as temperature changes.
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