Molality

Molality is a measure of the concentration of a solute in a solution in terms of amount of substance in a specified amount of mass of the solvent. This contrasts with the definition of molarity which is based on a specified volume of solution.

A commonly used unit for molality in chemistry is mol/kg. A solution of concentration 1 mol/kg is also sometimes denoted as 1 molal, although the ISO recommends against this usage.

Definition

Molality, m, is defined as the number of moles of solute, n, divided by the mass m of the solvent in kilograms:

m = n / m

This contrasts with the definition of molarity, M, which is defined as the number of moles of solute divided by the volume V of the solution in litres:

M = n / V

Properties

Molality is used in expressing the properties of dilute solutions and colligative properties, which depend on the ratio of the number of solute particles to the number of solvent molecules in a solution, and not on the type of chemical species present.

See also

• Molarity
• Mole (unit)
• Solution
• Colligative properties
• Dilute solution

References

Molality Resources
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