What chemical compounds might be present in drinking water?

# 3.2.2 Nomenclature of oxoacids

Traditionally oxoacid formulas are written with hydrogen first, which conceals the fact that hydrogen is often bonded to oxygen. The rarely used formula (OH)3PO is a more meaningful formulation of phosphoric acid than H3PO4. The nomenclature is further complicated by the fact that the inorganic acids and their organic derivatives also have different common names; for instance, phosphorous acid in inorganic chemistry becomes phosphonic acid for its organic derivatives. To help you through this topic the prefixes and suffixes are in bold italics and the most common name is always used.

Historically, where the central element forms oxoacids in two oxidation numbers:

- the higher state is indicated by the suffix - ic
- the lower state is indicated by the suffix - ous .

For example phosphoric acid, H3PO4, and phosphorous acid, H3PO3, have phosphorus oxidation numbers +5 and +3 respectively.

If more than two oxidation numbers are involved, the prefixes per - and hypo - are used as well:

- per - denotes the highest oxidation number
- hypo - denotes the lowest oxidation number.

For example the oxoacids of chlorine are shown in Table 4.

## Table 4 The oxoacids of chlorine.

Formula | Oxidation number | Name
--- | --- | ---
HClO4 | +7 | per chlor ic
HClO3 | +5 | chlor ic
HClO2 | +3 | chlor ous
HClO | +1 | hypo chlor ous

Oxoanions derived from - ic acids are given the ending - ate and from - ous acids are given the ending - ite .

To practise, name the following anions OCl−, ClO4 − and NO2 −.

## Answer

The ions are hypochlorite, perchlorate and nitrite.

Condensed forms of oxoacids are also distinguished by prefixes:

- ortho - refers to the 'monomeric', or most highly hydroxylated, form
- meta - refers to the 'polymeric', or least highly hydroxylated, form
- di - and tri - in this context logically refer to 'dimers' or 'trimers'.

The oxoanions follow this labelling convention, for example orthophosphates, PO4 3−, diphosphates, P2O7 4−, and triphosphates, P3O10 5−. Additionally, the prefix cyclo- or catena- distinguish cyclic from linear condensed anions, respectively.

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