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Molarity (M) is defined as moles of solute per liter of solution while Normality (N) is defined as moles of reacting units per liter of solution. Most texts use sulfuric acid as an example to demonstrate the difference: a 1 M solution of sulfuric acid is 2 N because each mole of sulfuric acid gives two moles of $\ce{H+}$ (the "reacting unit"). Why then is concentrated sulfuric acid labeled as 18 N when it should be 18 M and 36 N (according to my text's example above)?

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  • $\begingroup$ Just for reference (and archiving), can you give an example of a label of "18 N" for sulfuric acid? $\endgroup$ – Geoff Hutchison Mar 10 '15 at 16:00
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Who labels concentrated sulfuric acid as 18N? 36N sounds right to me and Sigma-Aldrich:

Concentration: The commercial reagent contains 93-98% H2SO4, the balance being water. Its effective concentration is 18 M (36N).

https://www.sigmaaldrich.com/content/dam/sigma-aldrich/docs/Sigma-Aldrich/Product_Information_Sheet/258105pis.pdf

IUPAC discourages use of normality because it is ambiguous. The number of equivalents can be ambiguous depending upon the purpose (equivalents of protons, equivalents of electrons, etc.).

See the following IUPAC document: http://old.iupac.org/publications/analytical_compendium/Cha06sec3.pdf

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  • $\begingroup$ Indeed, I'd guess "18 N" is probably a typo for "18 M" $\endgroup$ – Geoff Hutchison Mar 10 '15 at 16:00
  • $\begingroup$ Alternately, 18 N sulfuric acid is about 50 mol% sulfuric acid in water, which, depending on your view, is still pretty "concentrated". $\endgroup$ – Curt F. Mar 11 '15 at 12:34

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