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This is a really basic (perhaps trivial) question, but I just want to confirm:

What are the SI symbols for molar mass, molarity, and molality? For molality and molarity, there is a difference, but I’ve found that the symbol is the same for molarity and molar mass, i.e. M (a big m).

Also, what are the meaningful differences between molarity and molality? Does it just have to do with mass (i.e. difference in mass between solute and solvent) vs. concentration (i.e. within the solution)?

I get the sense that molarity is defunct? I read this here -

https://www.nist.gov/pml/special-publication-811/nist-guide-si-chapter-8

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    $\begingroup$ Have you heard of the pigeon hole effect? There are more quantities than English or Greek letters. You will always see some overlap. It is the context which is important. $\endgroup$
    – AChem
    Mar 21, 2021 at 0:11
  • $\begingroup$ That is such a good point, @M.Farooq. That is not clarified or emphasized enough. I get the sense that you can use M for molarity or molar mass; just specify beforehand, e.g. molar mass M of NH3 (or molarity M of NaCl). I see no reason to not use both, as long as you make it clear what the context is. $\endgroup$
    – Alex
    Mar 21, 2021 at 1:20
  • $\begingroup$ Exactly. I check IUPAC gold when I wish to confirm. You can see $M_r$ for relative molecular mass. If someone sticks to IUPAC recommendations then it is very clear who is who. $\endgroup$
    – AChem
    Mar 21, 2021 at 1:35
  • $\begingroup$ goldbook.iupac.org/terms/view/R05271 $\endgroup$
    – AChem
    Mar 21, 2021 at 1:35
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    $\begingroup$ Related on meta: Quantities and units $\endgroup$
    – Loong
    Mar 21, 2021 at 7:10

1 Answer 1

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According to IUPAC, the official name for molarity is amount of substance concentration, and its symbol is $c$. The symbol for molality is $m$ or $b$, and the symbol for molar mass is $M$.

what are the meaningful differences between molarity and molality?

Molality does not change with temperature because mass and chemical amount is temperature-independent. The amount of substance concentration (molarity) does because substances expand (mostly) with in increasing temperature. So for most applications where the temperature changes, molality is the better choice.

On the other hand, for room temperature applications where volumetric measurements are convenient, it is common to use molarity as the measure of concentrations.

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