Why are boric and phosphoric acids are formulated as:

  • H3BO3

  • H3PO4

Instead of:

  • HBO2

  • HPO3

Because the acids end with the -ic suffix, the valences of the elements between the hydrogens and oxygens are 3, 4 and 5 respectively, since Hydrogen has 1+ valence and the Oxygen 2-.

In the acids H3BO3 and HBO2, both of the borons have a valence of 3 +, (the multiplication of 1x3 plus 3x1 gives us 6 + which is the same result (with a different sign) of the multiplication of the valence of the Oxygen 2- and 3 (- 6) and in the acid HBO2 (1x1+3x1 = 4 +; - 2x2 = - 4). How can I know which one is the boric acid when the nomenclature is given? Of course I can memorize it but I don't really know why only one of them is the boric acid.


$HBO_2$ is metaboric acid (left pic.), boric is on the right pic.

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$HPO_3$ is metaphosphoric acid (on the left pic),...

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Actually, blame old nomenclature. Meta-acid has "one less $H_2O$ molecule" than ortho-acid. Thus, $HBO_2$ is metaboric acid, while $H_2BO_3$ is orthoboric (same about any other acid). But as you can see from pics, it's not really "water molecule" that differs meta-acid from ortho-acid. In case of metaboric acid, it's not even 1 H and 2 O's difference! So now ortho-acids are called simply acids (for example, boric), while meta-acids are still called meta-acids. Maybe because they are formed from ortho-acids. So, ortho-acids are considered "true acids", while meta-acids are, like, secondary. But I should mention that sometimes ortho-acids are called ortho-acids. I believe it depends on occurence of these substances in history of chemistry: the most "historically" important of two (or popular) has the right to skip prefix.

[remark] I know that metaphosphoric acid, as well as pic which I've given, is old fossil and now there is no such thing as "monophosphoric" acid (check for cyclo-triphosphoric acid, though). But the question was about this acid.

[edited]For borates, ortho-acid is almost always "boric acid" (rarely - "orthoboric acid"), and meta-acid is always "metaboric acid". So, (ortho)boric acid = H3BO3, metaboric acid = HBO2. Phosphoric acid = H3PO4. It's very unlikely to meet name "orthophosphoric", because you have no metaphosphoric pair. Sometimes (in old textbooks/in poor-made school course of chemistry) you can hear "metaphosphoric acid", which is on the pic above, but the name and pic itself are false. HPO3 is now considered to be cyclo-triphosphoric acid, as I've said. As for other possible acids - some have "true name" for meta-acid, while ortho-acids are called ortho-acids, some are called ortho- and meta-, some are like phosphoric acids: there is no ortho- or meta-, only "true acid" and cyclo-acid. Specificity of nomenclature.

Googling for any good explanation now gave me this: https://www.quora.com/What-is-the-meaning-of-ortho-pyro-and-meta-as-in-the-context-of-phosphoric-acids

Good and detailed info about IUPAC nomenclature. Hope this helps.

  • $\begingroup$ Oh God, that's a lot of information! Yes, I knew about the meta, ortho and the di prefixes. This creates a bit of a confusion for me. How do I exactly know which prefix to use when the formulation is given to me? Or if the nomenclature is referring to the ortho when it isn't written (without the Lewis structure)? I remember something like: for the valences that are even numbers you would have to add 1 molecule of water to the oxide, or something like that, in the meta prefixes. Could you give me an example on how to do that? (I don't know if I explained myself well enough) $\endgroup$ – Sirena S. C. Nov 30 '18 at 19:27
  • $\begingroup$ Please, check [edited] part. $\endgroup$ – Kelly Shepphard Nov 30 '18 at 19:59
  • $\begingroup$ I think I get it now, thank you! I will check the link. $\endgroup$ – Sirena S. C. Dec 1 '18 at 8:26

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