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Name: Darcy
Status: educator
Grade: other
Location: NE
Date: January 2008


Question:
When naming acids from anions from the polyatomic ion list we had trouble with the anions that begin with hydrogen. Like dihydrogen phosphate (H2PO4) plus hydrogen, is it H3PO4 which is the same formula as phosphoric acid? And is it called Dihydgrogen phosphoric acid?



Replies:
Darcy,

There are three rules for naming binary inorganic acids.

1) If the anion does not contain an oxygen, then the acid name follows the form: "hydro"+anion root name+"ic acid". Thus: HCl is named hydro+chlor+ic acid, and HCN would be named hydrocyanic acid.

2) If the anion does contain an oxygen and its name ends in "ite" then the acid name follows the form: anion root name+"ous acid". Thus HNO2 has the name nitr+ous acid and H2SO3 is called sulfurous acid (the "ur" being added for phonetic purposes).

3) If the anion does contain an oxygen and it name ends in "ate" then the acid name follows the form: anion root name+"ic acid". Thus HNO3 has the name nitr+ic acid, H2Cr2O7 is dichromic acid and H3PO4 is called phosphoric acid ("the "or" being added for phonetic purposes).

Greg (Roberto Gregorius)


Hi Darcy,

Oxoacids and their ions are named either using the ...ic acid / ate or ...ous acid / ite suffix connections. Names of intermediate ions (with one or more protons) must also indicate the number of protons.

For phosphate's ions, here are the IUPAC names: (using the super/subscript notation that water is H_2O, chloride is Cl^-):

H_3PO_4 is phosphoric acid (*not* trihydrogen phosphate)
H_2PO_4^- is dihydrogen phosphate
HPO_4^2- is hydrogen phosphate (*not* monohydrogen phosphate)
PO_4^{3-} is phosphate

In this convention, the completely protonated form is called X....ic acid and the completely deprotonated form is X....ate. For other species we indicate the number of hydrogens, unless there is only one hydrogen (because monohydrogen phospate is considered unnecessarily long and complicated). Another example:

H_2SO_4 is sulfuric acid (*not* dihydrogen sulfate)
HSO_4^- is hydrogen sulfate (*not* monohydrogen sulfate)
SO_4^{2-} is sulfate

Now, consider an ...ous acid / ite example:

H_2SO_3 is sulfurous acid
HSO_3^- is hydrogen sulfite
SO_3^2- is sulfite

Note that sulfate has more oxygens than sulfite but the same charge. This relationship holds for other oxyanions as well, but note that ...ate vs. ...ite only refers to the relative number of oxygens for a given atomic center (so for example, NO_3^- is nitrate and HNO_3 is nitric acid, but NO_2^- is nitrite and HNO_2 is nitrous acid).

Salts are named by giving the name of the cation followed by the name of the anion, without indicating directly the number of cations or anions. So for example, Na_3(PO_4) is properly named sodium phosphate (although it is called trisodium phosphate in hardware and paint stores), and Na_2(HPO_4) is properly named sodium hydrogen phosphate.

This can be done because Na^+ is the only possible charge state for sodium. If the cation is a transition metal (or a lanthanide/actinide for that matter), often more than one charge is possible and the charge of the cation must somehow be indicated, either using the Stock roman numeral system or the historic conventional names: FePO_4 is iron (III) phosphate (since the iron is Fe^{3+}) or ferric phosphate; Fe_3(PO_4)_2 is iron (II) phosphate (since the iron is Fe^{2+}) or ferrous phosphate (Note that we do not call the first compound monoferric monophosphate or the second compound triferrous diphosphate; those are real jawbreakers and unnecessarily complicated). Some transition metals have only one charge state possible (like Ag^+) and for those cases we revert to the same method as used for sodium above (Ag_3(PO_4) is silver phosphate, etc.).

Hope this helps!
Dr. Topper



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