Molality

Definition

Concentration as moles of solute per kilogram of solvent: the temperature-proof alternative to molarity.

$m = \frac{n}{\text{kg solvent}}$

Symbol: $m$ · Unit: mol/kg

Technical definition

Molality (m) is moles of solute per kilogram of solvent. Unlike molarity it is temperature-independent, because masses do not expand, making it the unit of choice for colligative property calculations.

Example

Dissolving 1 mol of NaCl in exactly 1 kg of water gives a 1 m solution regardless of temperature.

Common confusion

Molality and molarity are different units with different denominators. Molality divides by kg of SOLVENT, molarity by L of SOLUTION. For dilute aqueous solutions they are numerically close but never identical.