molality
/ moh-LAL-ih-tee /
Picture weighing your water on a balance instead of pouring it into a measuring jug. That swap — from volume to mass — is the whole difference between molality and its better-known cousin, molarity.
Molality (unit mol/kg, written with a lowercase m) is the number of moles of solute per kilogram of solvent. Because both moles and kilograms are counted by weighing, and mass does not change when you heat a sample, molality stays exactly the same at every temperature.
That temperature-proof quality is why molality shows up in physical chemistry — in boiling-point elevation, freezing-point depression, and other 'colligative' effects that demand a count of particles unaffected by thermal expansion. For everyday bench work, though, volume is easier to measure, so molarity usually wins.
Adding 1 mole of glucose to 1 kilogram of water makes a 1 mol/kg (1 m) solution; heat it or chill it and the molality is unchanged, even though its volume — and hence its molarity — shifts.
Molality is mass-based, so it ignores temperature entirely.
Watch the spelling: molality (kg of solvent) versus molarity (L of solution). They look almost identical but are physically different quantities.