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Inorganic Acid Names and Formulae Introductory Chemistry Tutorial

Key Concepts

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Binary Acids (hydrohalic acids)

Hydrohalic acids are aqueous solutions of binary inorganic compounds in which hydrogen, H, is combined with a halogen (Group 17) element.

  Names and Symbols of Group 17 Elements
Name of halogen
(Group 17 element)
fluorine chlorine iodine bromine astatine
Chemical Symbol F Cl I Br At

The general molecular formula for all hydrohalic acids is HX(aq), where H is the symbol for hydrogen, X represents the symbol for one of the halogen elements and (aq) indicates that this is an aqueous solution.
Note that the chemical symbol for hydrogen, H, is written before the chemical symbol of the Group 17 element.
The formula for each hydrohalic acid you are likely to encounter is given in the table below:

  Hydrohalic acids(4)
Molecular Formula HF(aq) HCl(aq) HI(aq) HBr(aq)

The name of hydrohalic acids always have the prefix "hydro", representiong the acidic hydrogen, followed by the name of the halogen with final "ine" of the name substituted with "ic", then followed by the separate word "acid".

molecular
formula
prefix + modified name
of element
+ acid = "acid" name
HF(aq) hydro + fluorine + ic + acid = hydrofluoric acid
HCl(aq) hydro + chlorine + ic + acid = hydrochloric acid
HBr(aq) hydro + bromine + ic + acid = hydrobromic acid
HI(aq) hydro + iodine+ ic + acid = hydroiodic acid

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Oxoacids (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 relative positions of hydrogen (H), oxygen (O) and the central element (E) are shown below:(5)

possible
structure
HO-E=O
 
 O
||
 
HO-E-O
 
 O
||
 
HO-E-OH
 
 O
|
 
HO-E-OH
 |
O
 
 
 O
||
HO-E-OH
 |
OH
molecular
formula
HEO2   HEO3   H2EO3   H2EO4   H3EO4

Molecular formulae are given in the classical oxoacid format with the "acid" or "replaceable" hydrogen atoms (oxygen-bound hydrogen atoms) listed first, followed by the central atom (E), and then the oxygen atoms (O).

Oxoacids are named with the name of the central element first using a modified ending (suffix) to indicated the relative amount of oxygen present, followed by the word "acid".

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. (6)

elements in acid
oxygen, hydrogen and ...
most oxygen (ic)
(central atom in highest oxidation state)
least oxygen (ous)
(central atom in lowest oxidation state)
boron boric acid
H3BO3
 
carbon carbonic acid
H2CO3
 
silicon silicic acid
H4SO4
 
nitrogen nitric acid
HNO3
nitrous acid
HNO2
phosphorus phosphoric acid
H3PO4
phosphorous acid
H3PO3
arsenic arsenic acid
H3AsO4
arsenous acid
H3AsO3
antimony antimonic acid
H3SbO4
antimonous acid
H3SbO3
sulfur sulfuric acid
H2SO4
sulfurous acid
H2SO3
selenium selenic acid
H2SeO4
selenous acid
H2SeO3
tellurium telluric acid
H2TeO4
tellurous acid
H2TeO3

Oxoacids (oxyacids) in which the central atom is a halogen (Group 17) element (X) can be one of 4 different compounds so the prefixes "per" and "hypo" are used in addition to the suffixes "ic" and "ous" to denote relative amounts of oxygen in the compound:

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 but more oxygen than hypohalous acid and has the general molecular formula HXO2

hypohalous acid has the least oxygen of all and has the general molecular formula HXO

Name and Molecular Formula of Oxoacids with a Central Halogen Atom
elements in acid
oxygen, hydrogen and...
most oxygen
(central atom in highest oxidation state)
less oxygen even less oxygen least oxygen
(central atom in lowest oxidation state)
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
iodine periodic acid
HIO4
iodic acid
HIO3
iodous acid
HIO2
hypoiodous acid
HIO

And this allows us to end this page with a chemistry joke or two:

Question: What do you call an acid that only works some of the time?
Answer: periodic acid (mispronunciation of periodic acid)

Question: What acid is prepared by boiling up old periodic tables?
Answer: periodic acid (mispronunciation of periodic acid)

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Footnotes: reference "Nomenclature of Inorganic Chemistry: IUPAC Recommendations 2005" (Red Book)

(1) IUPAC is the acronym for the International Union of Pure and Applied Chemistry.
IUPAC reviews existing methods of naming compounds (called nomenclature) and makes recommendations on how compounds should be named in the future so that there is no ambiguity in the name (and formula) of any particular compound.

(2) IUPAC refers to these as oxoacids, but some syllabus documents use the term oxyacids.

(3) Does this sound like a form of systematic nomenclature to you?
It isn't really, mainly because the suffixes "ic", "ous" are not used to refer to a particular amount of oxygen (or a particluar oxidation state of the central atom) and because the prefixes "per" and "hypo" are not used in a systematic way.
Therefore these "acid names" are not truly based on either the composition or the structure of the compounds.

(4) What happened to astatine? Astatine is highly radioactive and has not been found occurring naturally on Earth so its chemistry is not well understood. Using the principles of periodicity, the trend in the properties of group 17 elements of the periodic table leads us to expect its properties to be similar to iodine.

(5) The structures here are for demonstration of the central element (E) and possible arrangements of oxygen (O) and hydrogen (H) atoms, they should NOT be used as general structures in which you can just replace the "E" with the symbol of an element.

(6) Note that structurally each of the oxoacids must contain at least one OH group and this effects the IUPAC systematic additive names as shown below:

Acceptable common name
(systematic additive name)
Formula in oxoacid format
[formula as for coordination entity]
boric acid
(trihydroxidoboron)
H3BO3
[B(OH)3]
carbonic acid
(dihydroxidooxidocarbon)
H2CO3
[CO(OH)2]
cyanic acid
(hydroxidonitridocarbon)
HOCN
[C(N)OH]
silicic acid
(tetrahydroxidosilicon)
H4SiO4
[Si(OH)4]
nitric acid
(hydroxidodioxidonitrogen)
HNO3
[NO2(OH)]
nitrous acid
(hydroxidooxidonitrogen)
HNO2
[NO(OH)]
phosphoric acid
(trihydroxidooxidophosphorus)
H3PO4
[PO(OH)3]
phosphorous acid
(trihydroxidophosphorus)
H3PO3
[P(OH)3]
arsenic acid, arsoric acid
(trihydroxidooxidoarsenic)
H3AsO4
[AsO(OH)3]
arsenous acid, arsorous acid
(trihydroxidoarsenic)
H3AsO3
[As(OH)3]
antimonic acid, stiboric acid
(trihydroxidooxidoantimony)
H3SbO4
[SbO(OH)3]
antimonous acid, stiborous acid
(trihydroxidoantimony)
H3SbO3
[Sb(OH)3]
sulfuric acid
(dihydroxidodioxidosulfur)
H2SO4
[SO2(OH)2]
sulfurous acid
(dihydroxidooxidosulfur)
H2SO3
[SO(OH)2]
selenic acid
(dihydroxidodioxidoselenium)
H2SeO4
[SeO2(OH)2]
selenonic acid
(hydridohydroxidodioxidoselenium)
H2SeO3
[SeO(OH)2]
telluric acid
(dihydroxidodioxidotellurium)
H2TeO4
[TeO2(OH)2]
tellurous acid
(dihydroxidooxidotellurium)
H2TeO3
[TeO(OH)2]
perchloric acid
(hydroxidotrioxidochlorine)
HClO4
[ClO3(OH)]
chloric acid
(hydroxidodioxidochlorine)
HClO3
[ClO2(OH)]
chlorous acid
(hydroxidooxidochlorine)
HClO2
[ClO(OH)]
hypochlorous acid
(chloridohydridooxygen)
HClO
[O(H)Cl]
perbromic acid
(hydroxidotrioxidobromine)
HBrO4
[BrO3(OH)]
bromic acid
(hydroxidodioxidobromine)
HBrO3
[BrO2(OH)]
bromous acid
(hydroxidooxidobromine)
HBrO2
[BrO(OH)]
hypobromous acid
(bromidohydridooxygen)
HBrO
[[O(H)Br]
periodic acid
(hydroxidotrioxidoiodine)
HIO4
[IO3(OH)]
iodic acid
(hydroxidotrioxidoiodine)
HIO3
[IO2(OH)]
iodous acid
(hydroxidooxidoiodine)
HIO2
[IO(OH)]
hypoiodous acid
(hydridoiodidooxygen)
HIO
[O(H)I]