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


marked as duplicate by Wildcat, Jon Custer, Todd Minehardt, bon, ringo Oct 10 '16 at 19:06

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    $\begingroup$ Do you know what a mole is, to begin with? Also, welcome to Chem.SE. $\endgroup$ – Ivan Neretin Oct 9 '16 at 20:29
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    $\begingroup$ Related: Quick and simple explanation of molar mass, molecular mass and atomic mass $\endgroup$ – Loong Oct 9 '16 at 20:29
  • $\begingroup$ Because chemists defined it that way, following the KISS-principle. $\endgroup$ – Jan Oct 9 '16 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 '16 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 '16 at 20:39

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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