Why is molality important

Molality and molarity are both concentration terms. Given molarity's popularity molality seems rather antiquated. Why is molality still discused in modern chemistry classes? One of the key attractions of molality is that changing the temperature of a solution does not change the molality, while it may change the molarity.

The downside, of course, is that the amount of solute required to reach a given molality depends on the nature of the solvent; ambiguity may also arise when a mixed solvent system e. As for why it's introduced in school , that's a pedagogical question beyond the scope of this answer, but both molarity and molality have their uses in the laboratory.

Molality could be determined with respect to only the water, or the combined water-ethanol solution, leading to different calculated concentrations. In addition to JSK's excellent answer, I'd like to point out that there's a common pitfall related to molarity which JSKs answer might have slipped on : molarity is defined as amount of substance of solute per liter of solution , while molality is amount of substance of solute per kilogram of solvent.

This might not seem like a huge difference, but if you have a highly concentrated solution, you can sometimes get pretty big discrepancies between the volume of solvent and volume of solution.

Look at this question for an example of where the difference comes into play: the mistake would likely not have happened if molality was used as a measure, since doubling the volume of solute in a fixed volume of solvent doubles the molality, while it might do weird things to the molarity. Sign up to join this community. The best answers are voted up and rise to the top. Stack Overflow for Teams — Collaborate and share knowledge with a private group.

Create a free Team What is Teams? Learn more. What is the point of using molality when we have molarity? Units that appeared multiple times in a graph, a table, or in a sentence were counted as one use. Parts per million used to indicate NMR data were not counted.

Our results are shown in Figure 1. High Resolution Image. In this issue, molality was never used, whereas variants of molarity, such as nanomolar, were used times. We recognize that this survey is not exhaustive, and alternative procedures such as electronic searches could produce more precise counts. However, we do believe our results are indicative of the nonuse of molality outside of undergraduate education.

On the basis of our study, it appears that when publishing, chemists rarely use molality. If modern chemists do not use molality, why do we teach it?

Is it needed for the freezing point depression experiments? No, because it can easily be replaced with the fundamental units, mole solute per kilograms solvent.

Eliminating the discussion of molality would free up time to introduce the frequently used variants of molarity such as milli-, micro-, or nanomolar. Or considering the overly full general chemistry curriculum, any number of existing topics could be addressed more thoroughly and meaningfully. Supporting Information. Author Information. Nathan S. Winter - Department of Chemistry and Biochemistry, St. Cloud, Minnesota , United States ; Email: [email protected].

Faisal A. Omar - Department of Chemistry and Biochemistry, St. Benjamin L. Dreher - Department of Chemistry and Biochemistry, St. The authors declare no competing financial interest. General Chemistry Course Content J. A discussion is presented on what should and what should not be taught in general chem. The author argues that a common core needs to be established and presents an outline of subject material for a core curriculum.

The molality of our KCl and water solution is 1. Since the solution is very dilute, the molality is almost identical to the molarity of the solution, which is 1. We can also use molality to find the amount of a substance in a solution. For example, how much acetic acid, in mL, is needed to make a 3.

The moles of KCN can then be used to find the kg of acetic acid. We multiply the moles by the reciprocal of the given molality 3. The result is the desired mass of acetic acid that we need to make our 3 m solution:. Once we have the mass of acetic acid in kg, we convert from kg to grams: 0. Next, we use the density of acetic acid 1.

We must multiply by the reciprocal of the density to accomplish this:. Therefore, we require mL of acetic acid to make a 3. Boundless vets and curates high-quality, openly licensed content from around the Internet. This particular resource used the following sources:.