Oxyacid Nomenclature

To name oxyacids, you must first be able to recognize them by the general formula HaXbOc, with X representing an element other than hydrogen or oxygen. It will also be useful for you to know the names of the polyatomic oxyanions, because many oxyacid names are derived from them. If enough H+ ions are added to a (root)ate polyatomic ion to completely neutralize its charge, the (root)ic acid is formed. (See the table below.)

·  If one H+ ion is added to nitrate, NO3−, nitric acid, HNO3, is formed.

·  If two H+ ions are added to sulfate, SO42−, sulfuric acid, H2SO4, is formed.

·  If three H+ ions are added to phosphate, PO43−, phosphoric acid, H3PO4, is formed.

Note that the whole name for sulfur, not just the root, sulf-, is found in the name sulfuric acid. Similarly, although the usual root for phosphorus is phosph-, phosphor- is used in its place for oxyacids, such as phosphoric acid, that contain phosphorus.

Table Relationship between (root)ate polyatomic ions and (root)ic acids.

Oxyanion Formula / Oxyanion Name / Oxyacid Formula / Oxyacid Name
NO3− / nitrate / HNO3 / nitric acid
C2H3O2− / acetate / HC2H3O2 / acetic acid
SO42− / sulfate / H2SO4 / sulfuric acid
(Note that the whole name sulfur is used in the oxyacid name.)
CO32− / carbonate / H2CO3 / carbonic acid
PO43− / phosphate / H3PO4 / phosphoric acid
(Note that the root of phosphorus in an oxyacid name is phosphor-.)
ClO3− / chlorate / HClO3 / chloric acid
BrO3− / bromate / HBrO3 / bromic acid
IO3− / iodate / HIO3 / iodic acid
C2O42− / oxalate / H2C2O4 / oxalic acid
CrO42− / chromate / H2CrO4 / chromic acid

The names and formulas of (root)ate polyatomic ions are easily converted into names and formulas of the corresponding (root)ic acids, and vice versa.

·  If you know that chlorate is ClO3−, then chloric acid must be HClO3.

·  If you know that carbonic acid is H2CO3, the carbonate must be CO32−.

Just as certain elements form more than one oxyanion, they also form more than one oxyacid. Chlorine, for example, can form four oxyacids: HClO, HClO2, HClO3, and HClO4. The names for these can be determined from the name and formula for the (root)ic acid and the convention described below. For example, if you know HClO3 is chloric acid, you can use the following rules to figure out the names of HClO4, HClO2, and HClO.

·  An oxyacid with one more oxygen than the (root)ic acid will be named by writing per-, then the root of the name for the element other than hydrogen and oxygen, then -ic, and then acid. Therefore, HClO4 is perchloric acid.

·  An oxyacid with one less oxygen atom than the (root)ic acid is named by writing the root of the name for the element other than hydrogen and oxygen, then -ous, and then acid. Therefore, HClO2 is chlorous acid.

·  An oxyacid with two less oxygen atoms than the (root)ic acid is named by writing hypo-, then the root of the name for the element other than hydrogen and oxygen, then -ous, and then acid. Therefore, HClO, is hypochlorous acid.

The table below summarizes this convention.

Table Convention for Naming Oxyacids

Relationship / General Name / Example Name / Example Formula
one more oxygen atom than (root)ic / per(root)ic acid / perchloric acid / HClO4
(root)ic acid / chloric acid / HClO3
one less oxygen atom than (root)ic / (root)ous acid / chlorous acid / HClO2
two less oxygen atoms than (root)ic / hypo(root)ous acid / hypochlorous acid / HClO