Naming Binary Acids and Oxyacids
Acids are named from the anion in the compound: binary acids (hydrogen and one other element) are named hydro- + stem + -ic acid, as in hydrochloric acid (HCl), and oxyacids (hydrogen, oxygen and another element) are named from the oxyanion, with -ate becoming -ic acid and -ite becoming -ous acid, as in sulfuric acid (H2SO4) and sulfurous acid (H2SO3). In brief: HCl gas is hydrogen chloride, but HCl dissolved in water is hydrochloric acid; the names of oxyacids come from -ate and -ite ions: nitrate gives nitric acid and nitrite gives nitrous acid.
Acids are among the most important chemicals in laboratories, industry and living organisms. Their names follow patterns that are easy to learn when they are connected with the names of anions. This article explains the rules for binary acids and oxyacids, lists common acids with formulas, shows how to go from the formula to the name and back, works through examples including the calculation of concentrated acids, and gives a short history.
How are binary acids named?
A binary acid contains hydrogen and one nonmetal. The name of the aqueous solution is made from the prefix hydro-, the stem of the nonmetal and the ending -ic, followed by the word “acid”. Without water, the pure compound is a molecular compound and has a different name.
- HCl(g) is hydrogen chloride, and HCl(aq) is hydrochloric acid.
- HF(aq) is hydrofluoric acid, HBr(aq) is hydrobromic acid and HI(aq) is hydroiodic acid.
- H2S(aq) is hydrosulfuric acid.
- HCN(aq) is hydrocyanic acid, although cyanide is a polyatomic ion with the same naming pattern.
The state symbol (aq) is important. It tells us that the compound is dissolved in water and acts as an acid.
How are oxyacids named?
An oxyacid contains hydrogen, oxygen and a third element. The name is based on the oxyanion that is formed when the acid loses its hydrogen ions, and the prefix hydro- is not used.
- An anion ending in -ate gives an acid ending in -ic. Sulfate (SO42−) gives sulfuric acid (H2SO4).
- An anion ending in -ite gives an acid ending in -ous. Sulfite (SO32−) gives sulfurous acid (H2SO3).
- The prefixes per- and hypo- stay in the name: perchlorate gives perchloric acid, hypochlorite gives hypochlorous acid.
The stem of the name may change for some elements, as in sulfur (sulfuric) and phosphorus (phosphoric).
Common acids and their anions
| Acid formula | Name | Anion | Anion name |
|---|---|---|---|
| HCl | hydrochloric acid | Cl− | chloride |
| HNO3 | nitric acid | NO3− | nitrate |
| HNO2 | nitrous acid | NO2− | nitrite |
| H2SO4 | sulfuric acid | SO42− | sulfate |
| H2SO3 | sulfurous acid | SO32− | sulfite |
| H3PO4 | phosphoric acid | PO43− | phosphate |
| H2CO3 | carbonic acid | CO32− | carbonate |
| CH3COOH | acetic acid | CH3COO− | acetate |
| H2C2O4 | oxalic acid | C2O42− | oxalate |
| H2CrO4 | chromic acid | CrO42− | chromate |
The four oxyacids of chlorine
| Acid | Name | Anion | Oxidation number of Cl |
|---|---|---|---|
| HClO | hypochlorous acid | hypochlorite | +1 |
| HClO2 | chlorous acid | chlorite | +3 |
| HClO3 | chloric acid | chlorate | +5 |
| HClO4 | perchloric acid | perchlorate | +7 |
Which acids are strong?
The names give no information about the strength of an acid. The common strong acids, which are almost completely ionized in water, are hydrochloric (HCl), hydrobromic (HBr), hydroiodic (HI), nitric (HNO3), sulfuric (H2SO4, for the first ionization), perchloric (HClO4) and chloric (HClO3) acids. Most other acids, including hydrofluoric (HF), acetic, carbonic and phosphoric acid, are weak. A rough guide for oxyacids is that those with more oxygen atoms bonded to the central atom are stronger, so perchloric is stronger than chlorous acid.
Worked examples
- Name HBrO3. The anion is bromate (BrO3−), and “-ate” becomes “-ic”, so the name is bromic acid.
- Name H3PO3. The anion has one oxygen less than phosphate, so it is phosphite, and the acid is phosphorous acid.
- Formula of hydrofluoric acid. The prefix hydro- shows a binary acid, so it is HF(aq).
- Formula of perchloric acid. Perchlorate is ClO4−, so the acid needs one H+: HClO4.
- Name from the anion. The acid of the chromate ion CrO42− needs two hydrogen ions: H2CrO4, chromic acid.
- Concentration of concentrated hydrochloric acid. A bottle contains 37% HCl by mass with a density of 1.18 g/mL. In 1.000 L, the mass is 1,180 g, and the mass of HCl is 0.37 × 1,180 = 436.6 g. The moles are 436.6 ÷ 36.46 = 11.97, so the concentration is about 12.0 mol/L.
- Concentrated sulfuric acid. It is 98% by mass with a density of 1.84 g/mL. In 1.000 L, the mass is 1,840 g, and H2SO4 is 0.98 × 1,840 = 1,803 g, which is 1,803 ÷ 98.08 = 18.4 mol. The concentration is 18.4 mol/L.
How did acid names develop?
The word “acid” comes from the Latin acidus, meaning sour. Old names such as “oil of vitriol” (sulfuric acid), “spirit of salt” (hydrochloric acid) and “aqua fortis” (nitric acid) reflected the methods of preparation. Lavoisier believed that all acids contain oxygen and named oxygen from the Greek for “acid-former”. Humphry Davy showed in 1810 to 1815 that hydrochloric acid contains no oxygen, which led to the introduction of the hydro- prefix for acids without oxygen. The names of other oxyacids with -ic and -ous endings follow the 1787 system of Lavoisier and his colleagues.
Which mistakes are common?
Mistake 1: Using hydro- for oxyacids
Sulfuric acid does not take the prefix hydro-. It is only for binary acids.
Mistake 2: Confusing hydrogen chloride and hydrochloric acid
The first is the pure gas. The second is its aqueous solution. The (aq) symbol shows which is meant.
Mistake 3: Mixing -ic and -ous
-ic acids come from -ate anions (more oxygen). -ous acids come from -ite anions (less oxygen).
Mistake 4: Assuming that the name shows the strength
Names show composition only. Hydrofluoric acid is weak, while hydrochloric acid is strong.