Molality Calculator

In short: Compute molality from solute mass, molecular weight, and solvent mass. It is free, works in your web browser and needs no download.

Compute solution molality from solute mass, molecular weight, and solvent mass.

How molality is calculated

Molality (m) is moles of solute per kilogram of solvent: m = (mass in g ÷ molecular weight in g/mol) ÷ solvent mass in kg. Example: 1.50 g of NaCl (58.44 g/mol) in 100 g of water gives 0.02567 mol ÷ 0.100 kg = 0.257 mol/kg.

Because it uses mass rather than volume, molality does not change with temperature. It is the unit used for colligative properties such as boiling point elevation and freezing point depression.

Example use: the freezing-point depression of a solution is ΔT = i × K_f × m, where K_f for water is 1.86 °C·kg/mol and i is the van 't Hoff factor (about 2 for NaCl). A 0.257 mol/kg NaCl solution therefore freezes roughly 0.96 °C below pure water, ignoring non-ideal behaviour. Weigh the solvent rather than measuring its volume.