The formula
m = n / kg
| Symbol | Quantity | Unit |
|---|---|---|
| m | Molality | mol/kg |
| n | Moles of solute | mol |
| kg | Mass of solvent | kg |
What it means
Molarity is measured per litre of solution and molality per kilo of solvent, and the distinction is not a notational fussiness: **molality does not change with temperature**. A volume expands when heated, so a 1 molar solution stops being one if you warm it; a mass does not expand, so a 1 molal solution stays 1 molal at any temperature. That is why colligative properties — freezing point depression, boiling point elevation — are always worked out with molality.
How to work it out by hand
- Work out the moles of solute from its mass and molar mass
- Weigh the solvent in kilos, not counting the solute
- Divide the moles by the kilos
What is worth knowing
In dilute aqueous solutions molarity and molality are nearly identical, because a litre of water weighs nearly a kilo and the solute barely changes the volume. That is why general chemistry uses them almost interchangeably and nobody notices the problem. As soon as the solution is concentrated, or the solvent is not water, they part company quickly: a 50 % sulfuric acid solution has a considerably higher molarity than molality, because its density exceeds 1.4. Rule of thumb: if the temperature is going to change or the solution is concentrated, use molality.