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Molality uses the mass of solvent, not total solution volume: m = n / kg solvent.
Formula and assumptions
m = n / kg solvent
Molality is based on the mass of solvent rather than the volume of solution, so it is useful when solution volume changes with temperature.
Assumption: The denominator is solvent mass, not total solution mass.
Variables and units
Worked example
0.50 mol of solute dissolved in 0.50 kg of solvent gives a molality of 1.00 mol/kg.
Molality versus molarity
Dissolving 10.0 g of glucose (180.16 g/mol, 0.0555 mol) in 200 g of water gives a molality of 0.278 mol/kg. Molality divides by the mass of solvent; molarity divides by the volume of solution. Because mass does not change with temperature, molality is used for boiling-point elevation and freezing-point depression.
| Molarity (M) | Molality (m) | |
|---|---|---|
| Divides by | litres of solution | kilograms of solvent |
| Changes with temperature | yes (volume expands) | no |
| Typical use | titrations, lab solutions | colligative properties |
For dilute aqueous solutions near room temperature the two values are close, because 1 L of water has a mass of about 1 kg.
Important limits
This does not calculate activity coefficients or account for chemical reactions.
Reference and related tools
IUPAC: molality. Return to Chemistry Calculators for the other tools in this group.