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*

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.

  • 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

(root)ic acid

chloric acid

HClO

 

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.

 

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