Why Is Molality Used Instead of Molarity?

When You Should Use Molality Rather Than Molarity

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

Question: When is molality used instead of molarity? Why is it used?

Answer: 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 this situations, molality is the preferred method of expressing concentration because the mass of solute and solvent in a solution does not change.

In particular, molality is used when you:

Use molality any time you expect the solute may interact with the solute. Use molarity for dilute aqueous solutions held at a constant temperature.

More About the Difference Between Molality and Molarity

Format
mla apa chicago
Your Citation
Helmenstine, Anne Marie, Ph.D. "Why Is Molality Used Instead of Molarity?" ThoughtCo, Mar. 6, 2017, thoughtco.com/differences-between-molality-and-molarity-609192. Helmenstine, Anne Marie, Ph.D. (2017, March 6). Why Is Molality Used Instead of Molarity? Retrieved from https://www.thoughtco.com/differences-between-molality-and-molarity-609192 Helmenstine, Anne Marie, Ph.D. "Why Is Molality Used Instead of Molarity?" ThoughtCo. https://www.thoughtco.com/differences-between-molality-and-molarity-609192 (accessed November 23, 2017).