Formula of Oxoacids

The formula of an oxoacids can be written from the formula of the corresponding oxyanion and the sufficient number of hydrogen ions (H⁺) so as to neutralise the charge on it.

Formula of the oxoanion + n H⁺ →  H_n(oxoanion)

For example, the formula of nitric acid can be written as follows:

Nitric acid contains oxyanion nitrate (NO₃^–).

Nitrate ion (NO₃^–) carries one negative charge on it. So, it requires only one hydrogen ion to neutralise its charge.

Therefore, the formula of the oxoacid containing NO₃^– ion = H⁺ + NO₃^– ® HNO₃

*In the case of organic acids, H adds to the COO^– group at the end of the molecule.

Naming Oxoacids

The general formula of oxoacids is H_aX_bO_c where a, b and c are small whole numbers (e.g. 1, 2, 3 …., etc.) and X is an element other than hydrogen and oxygen. Thus, oxoacids contain hydrogen and the corresponding oxyanion. 

Some examples are given below:

The name of an oxoacid can be written as follows:

·         For an acid containing the most common polyatomic ion of its group, simply use the first part of the polyatomic name and follow with the suffix ic and add the term acid at the end.

For example,

Name of HNO₃    =     Nitr         + ic + acid ⇒ Nitric acid

Name of HClO₃   =     Chlor             + ic + acid ⇒ Chloric acid

Name of H₂SO₄   =     Sulphur     + ic + acid ⇒ Sulphuric acid

Name of H₃PO₄   =     Phosphor   + ic + acid ⇒ Phosphoric acid

·         For the acid containing the polyatomic ion with the less oxygen than the ic, use the suffix ous, and add the term acid at the end.

For example,

Name of HNO₂    =     Nitr         + ous + acid ⇒   Nitrous acid

Name of HClO₂   =     Chlor             + ous + acid ⇒   Chlorous acid

Name of H₂SO₃   =     Sulphur     + ous + acid ⇒   Sulphurous acid

Name of H₃PO₂   =     Phosphor   + ous + acid ⇒   Phosphorus acid

Notice that the whole name for sulphur, not just the root, sulph-, is found in the name sulphuric acid.

Similarly, although the usual root for phosphorous is phosph-, the root phosphor- is used for phosphorus-containing oxoacids, as in the name phosphoric acid.

·         For and acid containing the polyatomic with two less oxygens than the ic, use the prefix hypo and the suffix ous, and add the term acid at the end.

For example,

Name of HClO  = Hypo + chlor + ous + acid    ⇒    Hypochlorous acid

Name of HPO₂  = Hypo + phosphor + ous + acid ⇒   Hypophosphorous acid

·         For an acid containing the polyatomic with one more oxygen than the ic, use the prefix per and the suffix ic and add the term acid at the end.

For example,

Name of HClO₄ = Per + chlor + ic + acid ⇒    Perchloric acid

Name of H₂SO₅ = Per + sulphur + ic + acid ⇒   Persulphuric acid

.