Acids: Chemical Formulas and Naming Rules, Part 2

The prefix hydro- means the acid contains no oxygen. The iodide ion, I⁻, has a 1− charge, so one hydrogen atom is enough: HI.

Citric acid is C₆H₈O₇. Only three of its eight hydrogen atoms are acidic: the ones in its three –COOH groups.

Vitamin C is ascorbic acid. Lactic acid is made by bacteria that sour milk and turn it into yogurt. Rhubarb leaves are poisonous because of their oxalic acid. Aspirin (acetylsalicylic acid) is made from salicylic acid. Malic acid is found mainly in apples.

Arsenic acid, H₃AsO₄, comes from the arsenate ion, AsO₄³⁻, so it has three acidic hydrogen atoms. Arsenic is in the same group as phosphorus, so the two elements form acids with matching formulas.

The -ate ion gives the -ic acid, and a 2− charge needs two hydrogen atoms: H₂SeO₄. It matches sulfuric acid, H₂SO₄, because selenium and sulfur are in the same group.

Oxalic acid has two –COOH groups, so it releases two H⁺ ions. Hydroselenic acid contains no oxygen, and selenide (Se²⁻) needs two hydrogen atoms. Benzoic acid releases only the hydrogen of its –COOH group, and lactic acid (C₃H₆O₃) also releases just one: the number of H⁺ ions depends on acidic groups, not on the number of hydrogen or oxygen atoms.

Formic acid has one carbon atom, acetic acid two, propionic acid three and butyric acid four. Butyric acid gives rancid butter its smell; its name comes from the Latin word for butter.

Aqua regia ("royal water") is a mixture of concentrated hydrochloric and nitric acids, usually in a 3:1 ratio. Neither acid can dissolve gold on its own.

Burning coal and gasoline releases sulfur dioxide and nitrogen oxides, which turn into sulfuric and nitric acids in the air. Even clean rain is slightly acidic because of carbonic acid from carbon dioxide, but it is much weaker.

Replacing one oxygen atom of H₂SO₄ with sulfur gives H₂S₂O₃. Its salt, sodium thiosulfate, was used to fix photographs.

HOCN is cyanic acid; swapping its oxygen for sulfur (thio-) gives thiocyanic acid, HSCN. H₄SiO₄ is silicic acid and H₂Cr₂O₇ is dichromic acid, with two chromium atoms. HCN contains no oxygen, so it cannot be cyanic acid and has no pair here.

Prussic acid is hydrocyanic acid, HCN. The Swedish chemist Carl Wilhelm Scheele first made it in 1782 from Prussian blue, which is how it got its name.

Formic acid is HCOOH: one hydrogen atom attached to a –COOH group. It can also be written as HCO₂H or CH₂O₂.

An -ic acid gives an -ate ion, an -ous acid gives an -ite ion, and a hydro-…-ic acid gives an -ide ion. Losing one H⁺ from sulfuric acid gives the hydrogen sulfate ion, HSO₄⁻; sulfite (SO₃²⁻) comes from sulfurous acid.

Sulfuric acid is the most produced industrial chemical in the world, at over 200 million tons a year. Most of it goes into making phosphate fertilizers.

Going down the halogen group, the bond between hydrogen and the halogen gets weaker, so H⁺ is released more easily. Hydrofluoric acid is a weak acid because the H–F bond is very strong, while hydroiodic acid is the strongest of the four.