I am completely confused about why molar mass is the same as atomic mass. Could someone help explain why, thoroughly, but in simplified terms?

  • 1
    $\begingroup$ Do you know what a mole is, to begin with? Also, welcome to Chem.SE. $\endgroup$ Oct 9, 2016 at 20:29
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    $\begingroup$ Related: Quick and simple explanation of molar mass, molecular mass and atomic mass $\endgroup$
    – user7951
    Oct 9, 2016 at 20:29
  • $\begingroup$ Because chemists defined it that way, following the KISS-principle. $\endgroup$
    – Jan
    Oct 9, 2016 at 20:35
  • $\begingroup$ yeah i understand that a mole the number of atoms in 12 grams of C-12 and thanks. $\endgroup$
    – Anon
    Oct 9, 2016 at 20:35
  • $\begingroup$ Ok so I know that the atomic mass of a C-12 atom is 12 u but how is the molar mass 12 g/mole then? $\endgroup$
    – Anon
    Oct 9, 2016 at 20:39

1 Answer 1


Ar - relative atomic mass is the ratio of the mass of one atom of an element to 1/12 of the mass of one atom Carbon-12 which has Ar of exactly 12. It is a dimensionless quantity. The Ar values are what you see in the periodic table, reflecting the isotopic compositions of the elements (that's why Ar(C)=12.011 and not 12).

Mr - relative molecular mass - ratio of the mass of a molecule to 1/12 of the mass of one atom Carbon-12. As with Ar, Mr is also a dimensionless quantity. It can be calculated as the sum of the relative atomic masses of the constituent elements.

M - molar mass - this is the mass of a substance (m) divided by the amount of substance (n) and therefore has units of g/mol. I can be calculated as Ar (for atoms) or Mr (for molecules) multiplied by 1 g/mol (also called the molar mass constant or standard molar mass) to ensure units of g/mol.


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