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 |
Oxyanion |
Oxyacid |
Oxyacid |
NO3- |
nitrate |
HNO3 |
nitric acid |
C2H3O2- |
acetate |
HC2H3O2 |
acetic acid |
SO42- |
sulfate |
H2SO4 |
sulfuric acid* |
CO32- |
carbonate |
H2CO3 |
carbonic acid |
PO43- |
phosphate |
H3PO4 |
phosphoric acid** |
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 |
*Note that the whole name sulfur is used in the oxyacid name.
**Note that the root of phosphorus in an oxyacid name is phosphor-.
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.
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If you know that chlorate is ClO3-, then chloric acid must be HClO3.
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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.
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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.
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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.
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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 |
Conversion of Names to Formulas for
Oxyacids
per(root)ic
acid
The formulas of the (root)ic acids can be determined from the formulas for the (root)ate polyatomic ions. If the name of the oxyacid is in the form per(root)ic acid, (root)ous acid, or hypo(root)ous acid, determine the formula from the formula of the (root)ic acid and the convention demonstrated in the table above.
EXAMPLE - Naming Oxyacids:
Write the names that correspond to the formulas HIO3, HIO, HC2H3O2
Solution: The first step in writing a name from a chemical formula is to decide which type of compound the formula represents. All three of these formulas represent oxyacids.
The name for IO3- is iodate, so HIO3 is iodic acid.
HIO has two fewer oxygen atoms than iodic acid, so it is hypoiodous acid.
The name of the oxyanion C2H3O2- is acetate, so HC2H3O2 is acetic acid. CH3CO2H and CH3COOH are also commonly used as formulas for acetic acid.
EXAMPLE - Formulas for Oxyacids:
Write the formulas that correspond to the names nitric acid and nitrous acid.
Solution: We recognize these as names of oxyacids, because they have the forms (root)ic acid and root(ous) acid.
Nitric acid is a very common acid, one whose formula, HNO3, you ought to memorize. You can also figure it out from the formula for nitrate, NO3-, by adding enough H+ ions to neutralize the charge. HNO3 is used to manufacture fertilizers and explosives and in the reprocessing of spent nuclear fuel.
You may not remember the formula of nitrous acid, but you can determine it from the formula for nitric acid, HNO3. The oxyacid with the name (root)ous acid has one less oxygen atom and the same number of hydrogen atoms as the (root)ic acid. Nitrous acid is HNO2