Molarity vs Molality: What’s the Difference?

Students often mix up molarity and molality. While both describe concentration, they differ in a crucial way.

Quick Definitions

Property	Molarity (M)	Molality (m)
Unit	mol/L	mol/kg
Based on	Volume of solution	Mass of solvent
Temperature dependent?	Yes	No
Most common use	Aqueous solutions	Colligative properties

Molarity (M)

Molarity = moles of solute ÷ liters of solution

Because it’s based on volume, molarity changes with temperature as liquids expand or contract.

Molality (m)

Molality = moles of solute ÷ kilograms of solvent

Because it’s based on mass, molality stays constant regardless of temperature changes.

When to Use Each

• Use molarity for most laboratory work, titrations, and reaction stoichiometry
• Use molality when studying colligative properties (boiling point elevation, freezing point depression, osmotic pressure)

Converting Between Them

For dilute aqueous solutions near room temperature, molarity and molality are approximately equal because the density of water is close to 1 kg/L. For concentrated solutions or non-aqueous solvents, the conversion requires knowing the solution density.

Use our molarity calculator to quickly compute concentration values.