M6-S2 Nomenclature of inorganic acids and bases
Nomenclature – Naming Acids
Acid names apply to the following two different groups of acids:
- binary acids do not contain oxygen (particularly hydrohalic acids).
- oxoacids (oxyacids) are inorganic compounds made up of oxygen.
Hydrohalic acids
- Hydrohalic acids are aqueous solutions of binary inorganic compounds in which hydrogen, H, is combined with a halogen (Group 17) element.
Molecular formular |
Prefix |
+ |
Modified name of element |
+ acid = |
“acid” name |
HF |
Hydro |
+ |
Fluorine + ic |
+ acid = |
Hydrofluoric acid |
HCl |
Hydro |
+ |
Chlorine + ic |
+ acid = |
Hydrochloric acid |
HBr |
Hydro |
+ |
Bromine + ic |
+ acid = |
Hydrobromic acid |
HI |
Hydro |
+ |
Iodine + ic |
+ acid = |
Hydroiodic acid |
Oxyacids
- Oxoacids (or oxyacids) are inorganic compounds made up of oxygen (O), hydrogen (H) and one other element (E) called the central atom or central element.
- Examples of molecular formula and their corresponding possible structures showing the general relative positions of hydrogen (H), oxygen (O) and the central element (E) are shown below:
- Oxoacids are named with the name of the central element first using a modified ending (suffix) to indicate the relative amount of oxygen present, followed by the word "acid"

Non-halogenic oxyacids
- The "ic" suffix indicates more oxygen is present in the compound than for the "ous" suffix. The table below includes compounds containing oxygen and hydrogen and one other element that is not a halogen (Group 17) element.
Table: naming nomenclature of oxyacids where the central atom is not a halogen.
Central element in oxyacid |
Most oxygen (highest oxidation state) |
Least oxygen (lowest oxidation state) |
Nitrogen |
Nitric acid (HNO3) |
Nitrous acid (HNO2) |
Phosphorus |
Phosphoric acid (H3PO4) |
Phosphorous acid H3PO3 |
Sulfur |
Sulfuric acid (H2SO4) |
Sulfurous acid (H2SO3) |
Halogenic oxyacids
- perhalic acid has the most oxygen of all with the general molecular formula HXO4
- halic acid has less oxygen than perhalic acid and has the general molecular formula HXO3
- halous acid has less oxygen than halic acid has the general molecular formula HXO2.
- hypohalous acid has the least oxygen of all and has the general molecular formula HXO
Table: naming nomenclature of oxyacids where the central atom is a halogen.
Central element in oxyacid |
More oxygen (highest oxidation state) |
Less oxygen (lowest oxidation state) |
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Chlorine |
Perchloric acid (HClO4) |
Chloric acid (HClO3) |
Chlorous acid (HClO2) |
Hypochlorous acid (HClO) |
Bromine |
Perbromic acid (HBrO4) |
Bromic acid (HBrO3) |
Bromous acid (HBrO2) |
Hypobromous acid (HBrO) |
Common Acids and Bases
Examples of common acids and bases
Table: common examples of strong and weak acids
Strong acid |
Weak acid |
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Molecular formula |
Name |
Molecular formula |
Name |
HClO4 |
Perchloric acid |
H3PO4 |
Phosphoric acid |
HI |
Iodic acid |
HF |
Hydrofluoric acid |
HBr |
Hydrobromic acid |
CH3COOH |
Ethanoic acid (acetic acid) |
H2SO4 |
Sulfuric acid |
CH2OOH |
Methanoic aicd |
HCl |
Hydrochloric acid |
C6H8O7 |
Citric acid |
HNO3 |
Nitric acid |
C2H2O4 |
Oxalic acid |
Table: common examples of strong and weak bases
Strong base |
Weak base |
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Molecular formula |
Name |
Molecular formula |
Name |
NaOH |
Sodium hydroxide |
NH3 |
Ammonia |
KOH |
Potassium hydroxide |
NaHCO3 |
Sodium bicarbonate |
Ba(OH)2 |
Barium hydroxide |
CH3NH2 |
Methylamine |
Ca(OH)2 |
Calcium hydroxide |
(CH3CH2)2NH |
Diethylamine |
Acids ranked by their dissociation constant, Ka, in water
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Deprotonation/ionisation of acids
- Not all hydrogen atoms (protons) of an acidic species can be deprotonated or donated away.
- Some acidic species may be able to donate more than one proton per acid molecule.
- Monoprotic – an acid molecule can only donate one proton. E.g. HCl, HNO3, CH3COOH (acetic acid)
- Diprotic – an acid molecule can donate up to two protons. E.g. H2SO4
- Triprotic – an acid molecule can donate up to three protons. E.g. H3PO4
If there are more than one proton that could be donated, each has a different Ka value. This means diprotic and triprotic acids typically have multiple Ka and pKa values. Consequently, the strength of acidic nature of these protons varies.
- The number of ionisable protons does not indicate acid strength. A triprotic acid (e.g. H3PO4) can have lower Ka values than a monoprotic acid e.g. HCl.
Phosphoric acid(H3PO4) |
Sulfuric acid (H2SO4) |
Hydrochloric acid (HCl) |
Nitric acid (HNO3) |
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Triprotic |
Diprotic |
Monoprotic |
Monoprotic |
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Acidic Hydrogens
· Davy’s hydrogen theory did not account of the acidic nature of hydrogens in hydrogen-containing molecules. Not all hydrogens can be ionised or deprotonated. Hydrogen atoms that can be deprotonated is referred to as the acidic hydrogen.
· Evidently, the pKa scale can be as low as –8 and as high as 50. Technically, all hydrogens can be deprotonated but ones with extraordinarily high pKavalues are effectively not acidic because it is very unlikely for these hydrogens to be deprotonated.
· Polarity When all other factors are kept constant, acids become stronger as the X–H bond becomes more polar. The second-row nonmetal hydrides, for example, become more acidic as the difference between the electronegativity of the X and H atoms increases. HF is the strongest of these four acids, and CH4 (methane) is one of the weakest Brønsted-Lowry acids known.
When these compounds act as an acid, a H–X bond is broken to form H+ and X– ions. The more polar this bond, the easier it is to form these ions. Thus, the more polar the bond, the stronger the acid.
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· Atomic Radius of the X Atom At first glance, we might expect that HF, HCl, HBr, and HI would become weaker acids as we go down this column of the periodic table because the X-H bond becomes less polar. Experimentally, we find the opposite trend. These acids actually become stronger as we go down this column.
Acids become stronger as the X-H bond becomes weaker, and bonds generally become weaker as the atoms get larger as shown in the figure to the right.
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