Molality

What is this formula?

Molality expresses the amount of solute in moles per kilogram of solvent. Unlike molarity, molality depends on the mass of solvent, so it is not affected by temperature-related volume changes.

When to use it

Use this formula in solution chemistry, colligative property calculations, thermodynamics, freezing point depression, boiling point elevation, and cases where temperature changes make volume-based concentration less reliable.

Example

A chemist dissolves 0.20 mol of glucose in 0.500 kg of water.

Data:

n = 0.20 mol

ms = 0.500 kg

Formula:

m = n/ms

Substitution:

m = 0.20 / 0.500

Result:

m = 0.40 mol/kg

The solution has a molality of 0.40 mol/kg.

Applications

- Colligative property calculations

- Freezing point depression

- Boiling point elevation

- Osmotic pressure studies

- Thermodynamic solution analysis

- Analytical chemistry calculations