Apart from sequihydrate and hemihydrate (1.5 and 0.5 respectively), I can't find a way to write any other fractional hydrates. Wikipedia states:

The notation "hydrated compound⋅nH2O", where n is the number of water molecules per formula unit of the salt, is commonly used to show that a salt is hydrated. The n is usually a low integer, though it is possible for fractional values to occur. For example, in a monohydrate n = 1, and in a hexahydrate n = 6.

Wikipedia says that fractional values can occur, but it doesn't show how to write the hydrate's name if n is fractional. Is there a simple way of writing a hydrate where n is, for example, 3.5 or 4.333...?

  • $\begingroup$ For n = 2.5, it could be hemipenthydrate. And hemihepthydrate for n=3.5, $\endgroup$
    – Maurice
    Jun 24, 2022 at 19:45
  • 1
    $\begingroup$ No, I definitely wouldn't. $\endgroup$
    – Mithoron
    Jun 24, 2022 at 20:26

1 Answer 1


There are no IUPAC rules as to how to use multiplicative prefixes for fractional water content in crystallohydrates. From Nomenclature of Inorganic Chemistry, IUPAC Recommendations 2005 (the “Red Book”), section IR-5.5 Names of (formal) addition compounds [1, p. 80] (strong emphasis mine):

For addition compounds containing water as a component, the class name ‘hydrates’ is acceptable because of well established use, even though the ending ‘ate’ might seem to indicate an anionic component. For hydrates with a simple stoichiometry, names of the classical ‘hydrate’ type are acceptable, but rules have not been formulated for non-integer stoichiometries such as that in Example 12 below. […]



  1. $\ce{3CdSO4·8H2O}\quad$ cadmium sulfate—water (3/8)

You need to convert the reciprocal of $n$ expressed as a decimal fraction to the simple fraction (or the ratio consisting of natural numbers) and put it in braces after the names of corresponding individual constituents of (formal) addition compounds separated by ‘em’ dash. Few examples:

  • $\ce{2ZnO·3B2O3·3.5H2O}\quad$ zinc(II) oxide—boron(III) oxide—water (4/6/7)
  • $\ce{MgCl2·4.33H2O}\quad$ magnesium(II) chloride—water (3/13)
  • $\ce{CHCl3·2H2S·4.5H2O}\quad$ chloroform—hydrogen sulfide—water (2/4/9)


  1. IUPAC. Nomenclature of Inorganic Chemistry, IUPAC Recommendations 2005 (the “Red Book”), 1st ed.; Connelly, N. G., Damhus, T., Hartshorn, R. M., Hutton, A. T., Eds.; RSC Publishing: Cambridge, UK, 2005. ISBN 978-0-85404-438-2. (PDF)

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