Molality and molarity are both concentration terms. Given molarity's popularity molality seems rather antiquated. Why is molality still discused in modern chemistry classes?
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6$\begingroup$ This is by no means an answer (you'll just have to wait for one), but the way I see it: "Molarity" is convenient, "Molality" is invariant ;) $\endgroup$– paracetamolCommented May 28, 2017 at 6:49
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$\begingroup$ IMHO I think molality should be more fundamental than molarity. Molarity is used simply for convenience, eg: pH is defined considering molarity, but a lot of actual chemical properties depend on molality, like boiling point rise and lowering of freezing point. $\endgroup$– Pritt says Reinstate MonicaCommented May 28, 2017 at 13:49
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3$\begingroup$ @PrittBalagopal The proper way of defining pH is through molality. What do you mean by more fundamental? Such notions tend to be rather subjective (in my opinion). $\endgroup$– Linear ChristmasCommented May 28, 2017 at 14:40
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$\begingroup$ The answer is that molarity requires you to measure the final volume of the solution and varies with temperature. While molarity is great for labs as you can easily determine how much of a substance you are adding, it is limited to small batches. In industry it is not practical to measure the final volume of 5000 gallons precisely but mass is much more easily measured. So rather than use molarity, you can use molality to produce repeatable results in a production facility. In essence is the chemical engineers' fault. $\endgroup$– A.K.Commented Jun 24, 2018 at 0:34
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2 Answers
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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. This is because the volume of the solution changes as a result of expansion or contraction of the solvent upon changing the temperature, and thus the molarity changes, since $$M=\frac{n}{V}.$$
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.g. a water-ethanol mixture) is used, as the choice of which compound is the solute changes the molality of the resultant solution.1 Molarity avoids this problem by requiring only the volume of solvent.
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.
1 Consider, for example, a solvent made from 95% water and 5% ethanol. Molality could be determined with respect to only the water, or the combined water-ethanol solution, leading to different calculated concentrations.
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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.
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1$\begingroup$ Thanks for point this out; it is certainly worth mentioning. I also thought about including some of the other potential pitfalls when making solutions in my original answer, such as the potential for volume contraction, e.g. when making methanol/water solution (see J. Chem. Ed. 1929, 6(6), p 1144),. This, of course, further complicates the situation, but I omitted it as it was only peripherally related to the question at hand. $\endgroup$– JSKCommented May 28, 2017 at 12:59