⚗️ Molality Calculator
Enter the solute's formula and mass and the solvent mass to get the molality — moles of solute per kilogram of solvent.
Molality
0.2 m
mol/kg
Moles of solute
0.1 mol
Molar mass
58.44 g/mol
Molality is moles of solute per kilogram of solvent: b = (mass ÷ molar mass) ÷ kg solvent. Unlike molarity (per litre of solution), molality uses solvent mass, so it doesn't change with temperature — which is why colligative-property work uses it. 🔒 In your browser.
How the molality calculator works
Molality is b = moles of solute ÷ kilograms of solvent. The tool reads the solute's formula to find its molar mass, converts your solute mass to moles (mass ÷ molar mass), then divides by the solvent mass in kilograms. Because it uses the solvent's mass rather than the solution's volume, molality is independent of temperature.
Molality (mol/kg, symbol m or b) is easy to confuse with molarity (mol/L, symbol M). Molality is preferred for colligative properties — boiling-point elevation and freezing-point depression — precisely because mass doesn't expand or contract with temperature the way volume does.
Frequently asked questions
What is molality?
Molality is the number of moles of solute per kilogram of solvent (mol/kg). It measures concentration by the solvent's mass, so it doesn't change when temperature changes the volume.
How do you calculate molality?
b = moles of solute ÷ kg of solvent. Find the moles from the solute's mass and molar mass (mass ÷ molar mass), then divide by the solvent mass in kilograms. This tool gets the molar mass from the formula for you.
What is the difference between molality and molarity?
Molality is moles per kilogram of solvent (mol/kg); molarity is moles per litre of solution (mol/L). Molality uses mass and is temperature-independent; molarity uses volume, which expands with temperature. For dilute aqueous solutions the two are numerically close.
Why is molality used for colligative properties?
Because freezing-point depression (ΔT = Kf·b) and boiling-point elevation (ΔT = Kb·b) depend on the ratio of solute particles to solvent mass, which molality captures directly and which doesn't drift with temperature.
What does a formula like CaCl2 give?
The tool computes its molar mass (110.98 g/mol) and, with your masses, the molality of the compound. Colligative effects also depend on how many ions it dissociates into (the van't Hoff factor), handled separately in the freezing/boiling-point tools.