molality
/ moh-LAL-ih-tee /
Molarity counts moles per litre of solution, but a litre is a slippery thing — heat the flask and it expands, so the number quietly drifts. Molality fixes this by switching from volume to weight: it counts moles of solute per kilogram of solvent. Since a kilogram is a kilogram whether hot or cold, molality holds steady no matter the temperature.
In symbols, molality = moles of solute / kilograms of solvent, with units of mol/kg, written m and read "molal." Crucially the denominator is the mass of solvent alone (not the whole solution), and it is measured by weighing, not by volume. To make a 1-molal solution you dissolve one mole of solute into exactly one kilogram of solvent.
Molality is the natural choice whenever temperature must not interfere — above all in the colligative-property formulas for boiling-point elevation and freezing-point depression, which are written in molality precisely so they stay valid as the solution heats or cools. For everyday bench work at fixed temperature, the easier-to-measure molarity is usually preferred instead.
Dissolve 1 mole of glucose (180 g) in exactly 1 kg of water and you have a 1-molal solution — and it stays 1-molal whether you warm it or chill it.
Molality = moles of solute per kilogram of solvent — temperature-proof.
In dilute aqueous solutions molality and molarity are numerically close, because one litre of water weighs about one kilogram. They diverge as the solution gets concentrated or the solvent's density strays far from 1 kg per litre.