Why Is Molality Used Instead of Molarity?

The difference between them and the correct application for each term

Both molality and molarity are units of concentration.
Both molality and molarity are units of concentration. Molality is used when temperature changes are expected. Rafe Swan, Getty Images

Molality (m) and molarity (M) both express the concentration of a chemical solution. Molality is the number of moles of solute per kilogram of solvent. Molarity is the number of moles of solute per liter of solution. If the solvent is water and the concentration of solute is fairly low (i.e., dilute solution), molality and molarity are approximately the same. However, the approximation fails as a solution becomes more concentrated, involves a solvent other than water, or if it undergoes temperature changes that could change the density of the solvent. In these situations, molality is the preferred method of expressing concentration because the mass of solute and solvent in a solution does not change.

When to Use Molality and When to Use Molarity

Molality is used any time you expect the solute may interact with the solute and in the following situations:

Molarity is used for dilute aqueous solutions held at a constant temperature. In general, the difference between molarity and molality for aqueous solutions near room temperature is very small and it won't really matter whether you use a molar or molal concentration.