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

.

ACIDS

A hydrogen-containing compound which gives hydrogen ions (H₃O⁺ or H⁺) when dissolved in water, is called an acid.

Acids can be classified into two groups:

❖ Hydracids: The compounds of hydrogen with highly electronegative

(nonmetallic) elements other than oxygen, are called hydracids.

Examples: HCl, H₂S, HBr, etc. are some typical hydracids.

❖ Oxoacids: The acids which contain an oxygen-containing anion (called

oxoanion), are called oxoacids (earlier called oxyacids). Thus, oxoacids are the

compounds which contain hydrogen, oxygen and another nonmetallic element.

Examples: H₂SO₄, HNO₃, H₃PO₄, H₂CO₃, etc. are some typical oxoacids.

Formula of Hydracids

The formula of a hydracid can be written as follows:

•   Write the symbol of hydrogen (H).
•   Write the symbol or formula of the anion alongwith its valency present in the acid on the right of H.
•   Criss-cross their valency numbers.
•   Divide these numbers (written as subscripts) by a common factor, if required.

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

Naming Hydracids

Hydracids are obtained by dissolving the corresponding gas in water. These gases are named as covalent compounds.

Name of the gaseous hydracid = Hydrogen + Name of the anion of the second                                               element

For example,

Name of HCl(g) = Hydrogen + Chloride → Hydrogen chloride

Hydracids are named by writing hydro followed by the root of the name of the

second element, then –ic, and finally acid with a space (blank) between ic and acid.

Hydro + Root of the second element + ic + acid

The only exception is HI (aq) in which case o in hydro is dropped while writing its

name.

Some typical hydracids

The gaseous HCl, HBr, H₂S, etc. do not bear the names of acids. They are named as covalent compounds. A compound that dissolves in water to form an acid is called an acid anhydride (acid without water). Only the aqueous solutions of acid anhydrides are named as acids. Therefore,

• HCl(g) is called hydrogen chloride while HCl(aq) is called hydrochloric acid
• HCN(g) is called hydrogen cyanide while HCN(aq) is called hydrocyanic acid.

The distinction is naming the anhydrides and the acids is not critical for oxoacids, because all their anhydrides are different molecules. For example, the anhydride of H₂SO₄ is SO₃, not gaseous H₂SO₄. Thus, H₂SO₄ is always called sulphuric acid, not hydrogen sulphate.

Chemical Communication - Language of Chemistry

ISBN : 9789386824424

The basic objective of preparing this study material is to familiarise the beginners to perform the following:

• Write the symbols of various elements, formulae of elements and compounds and their names and vice-versa

• Describe the elements in terms of symbols, compounds in terms of formulae and chemical reactions in terms chemical equation or ionic equations

• Write chemical equations from the world equations and vice-versa

• Balance chemical equation (both molecular and ionic) by various methods

• Name Acids, Bases and Salts from their formulae and vice-versa

• Name simple organic compounds on the basis of IUPAC recommendations

• Test their understanding by working out a large number of questions included in the text