// chemistry › Concentration Units
Calculate molality m = (n×1000)/W: moles of solute per kilogram of solvent, with symbol legend and real-world examples.
m = \frac{n × 1000}{W}
Because it is per kilogram of solvent - a mass that does not change with temperature, unlike volume. So molality stays fixed when a solution is heated or cooled, which makes it the right choice for precise work and for freezing/boiling-point calculations.
Molality uses the mass of the solvent alone (the dissolving liquid before adding solute). Molarity uses the total volume of the finished solution. Mixing these up is a common error.
Most visibly in antifreeze and road salt, where lowering the freezing point depends on molality. Also in raising boiling points for cooking and industry. Anywhere freezing or boiling points are deliberately shifted.
Because 1000 g is exactly 1 kilogram, so 1 mole in 1 kg is 1 mol/kg by definition - a deliberately clean example. The 1000 in the formula does the grams-to-kilograms conversion.
For very dilute water solutions near 4°C they are nearly equal, because 1 litre of water weighs about 1 kg. As solutions get concentrated or use other solvents, the two values drift apart and must be converted using density.