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Recently, my course of inorganic chemistry has begun, and in the nomenclature of inorganic acids, our professor told that phosphoric acid is $\ce{H3PO4}$ and phosphorous acid (orthophosphorous acid) is $\ce{H3PO3}$.

He explained to us that usually, the oxyacid of the highest oxidation state end with the suffix -ic.

So my question is, even in $\ce{H5PO5}$, phosphorous exhibits its highest oxidation state, so why isn't it called phosphoric acid, instead of $\ce{H3PO4}$?

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    $\begingroup$ FWIW, it is called holophosphoric acid and it is hypothetical. $\endgroup$ – Nilay Ghosh Jun 14 at 5:04
  • $\begingroup$ So, the highest oxidation state acids arent supposed to end with -ic? $\endgroup$ – Jackfrost Jun 14 at 5:07
  • $\begingroup$ what are the rules for nomenclature then? $\endgroup$ – Jackfrost Jun 14 at 5:07
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$\ce{H3PO4}$ is called orthophosphoric acid$\ce{^{[3]}}$. The prefix ortho- generally applies to the highest hydroxylated acid know either in free state or in salts or esters etc. pyro- and meta- denotes successive stages of dehydration. No provision is made in I.U.C(International Union of Chemistry) rules for naming the highest possible hydroxylated acid if they are hypothetical in any way but they are designated by a prefix holo- proposed by the German commission. So, in this case $\ce{H5PO5}$ is called holophosphoric acid.

But this type of nomenclature has been declared obsolete (see: What do prefix ortho, meta and pyro mean in chemistry) and shouldn't be used. Also see: When to use -ous and -ic while naming acids.

Notes and References

  1. Inorganic Syntheses, Volume 2
  2. The nomenclature of mineral acids
  3. The name "orthophosphoric acid" can be used to distinguish $\ce{H3PO4}$ from other "phosphoric acids (phosphorus acid series)". Nevertheless, the term "phosphoric acid" often means $\ce{H3PO4}$; and that is the current IUPAC nomenclature. (Wikipedia)
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