What does $\mathrm{M}$ signify in this pH calculation? To me, it just seems to be a random variable.

For example: $\ce{[H+]} = 1.0\times10^{-7}~\mathrm{M}$

I am not sure what the $\mathrm{M}$ is supposed to mean?

  • $\begingroup$ M = Molarity = mol per Liter of solution $\endgroup$
    – A.K.
    Commented Oct 11, 2018 at 16:04

1 Answer 1


It is shorthand for molarity. More precisely the amount concentration. It is used as a unit and stands for mole per litre.

The IUPAC gold book writes:

amount concentration,
Amount of a constituent divided by the volume of the mixture. Also called amount-of-substance concentration, substance concentration (in clinical chemistry) and in older literature molarity. For entities B it is often denoted by $\ce{[B]}$. The common unit is mole per cubic decimetre ($\mathrm{mol\, dm^{-3}}$) or mole per litre ($\mathrm{mol\, L^{-1}}$) sometimes denoted by $M$.

In your example it just means the concentration of protons in solution is $\ce{[H+]} = 1.0\times 10^{-7}~\mathrm{mol/ l}$.

  • $\begingroup$ Martin, thanks. What do protons have to do with this considering this is regarding PH. Also, you have "mol/1" so meaning mole per 1 what exactly? $\endgroup$ Commented Apr 17, 2016 at 15:57
  • 1
    $\begingroup$ @Atticus283blink Please read the Wikipedia page on pH it answers all your questions. Please also read the answer carefully, it's states multiple times that it is mole per litre. The unit litre may be written as L or l. $\endgroup$ Commented Apr 17, 2016 at 16:10
  • $\begingroup$ "mol/l" is mole(s) per liter He used a lowercase l instead of the uppercase L. // On old typewriters there was no digit 1. you were expected to use the lowercase L. Most typefaces now try to have somewhat different forms for the digit 1 and the lowercase letter L. The digit 0 and the uppercase letter O are another problem. $\endgroup$
    – MaxW
    Commented Apr 17, 2016 at 20:40

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