Oxyacids




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.

Notice 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, like 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
(Notice that the whole name sulfur is used in the oxyacid name.)
CO32- carbonate H2CO3 carbonic acid
PO43- phosphate H3PO4 phosphoric acid
(Notice 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  

 

Click here to see Example #1.

 

Conversion of Names to Formulas for Oxyacids

You can recognize a name as representing an oxyacid, because it will have one of the following forms.

per(root)ic acid

(root)ic acid

(root)ous acid

hypo(root)ous 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.  

Click here to see Example 2.

Click here to see acid nomenclature exercises.

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