Molality Calculator
Enter the moles of solute and the mass of solvent to get the molality in mol/kg — the temperature-independent way to express concentration.
Concentration in mol/kg
Enter the moles of solute and the kilograms of solvent and the calculator returns the molality (b = moles ÷ kg) in mol/kg.
Use the solvent mass
Molality divides by the mass of the solvent alone — not the whole solution — so weigh the liquid before you dissolve anything into it.
What is molality?
Moles of solute per kilogram of solvent
This molality calculator turns two measurements — the moles of solute and the mass of solvent in kilograms — into the molality of a solution in mol/kg. Molality (symbol b, sometimes m) is a measure of concentration: the moles of dissolved substance per kilogram of the solvent it is dissolved in. Unlike molarity, which divides by the volume of the whole solution, molality divides by the mass of the solvent alone. That distinction matters because mass does not change with temperature while volume does, so molality stays the same whether the sample is hot or cold.
Enter the moles of solute and the kilograms of solvent to get the molality in mol/kg instantly.
Molality is the moles of solute divided by the mass of the solvent in kilograms.
b = moles of solute ÷ kilograms of solventSuppose you dissolve 0.5 mol of a solute into 2 kg of water. Divide the moles of solute by the kilograms of solvent: 0.5 ÷ 2 = 0.25 mol/kg. That is the molality. If your solute is given as a mass, first convert it to moles by dividing the grams by the molar mass; if your solvent is given in grams, divide by 1000 to get kilograms before you divide.
The formula is exact, but the inputs have to be the right quantities.
Solvent mass, not solution volume
Molality uses the mass of the solvent in kilograms — a quantity that does not change with temperature — which is its key advantage over molarity. Molarity instead divides by the volume of the whole solution, and that volume expands and contracts as the temperature changes. Always weigh the solvent alone (the liquid you dissolve into), never the finished solution, and make sure the solute is expressed in moles. Mixing up these quantities is the most common source of error.