The rules for how we name acids aren’t obvious, but there are only three of them. First, if the cation is H-plus, and the anion is something-ide, then the acid name is going to be hydro-something-ic acid. If the cation is H-plus, and the anion is something-ite, then the acid name is going to be something-ous acid. And if the cation is H-plus, and the anion is something-ate, then the acid is going to be something-ic acid.
Let’s look at a few examples to better understand this. Name the acids formed by the following aqueous solutions. First, an aqueous solution of hydrogen chloride. Well that cation in this compound is the hydrogen ion, H-plus. The anion is the chloride ion, Cl-minus. And since chloride ends with -ide, the name for this acid is going to be hydrochloric acid. Let’s look at a different example. Let’s look at the aqueous solution of H2SO3. Again, the cation in this is the hydrogen ion. The anion is the sulfite polyatomic ion, SO3-two-minus. And since sulfite ends with -ite, the name of our acid is going to be sulfurous acid. Let’s look at one more example, an aqueous solution of HNO3. The cation in this is still hydrogen. The anion is nitrate, NO3-minus. And since that ends with -ate, the name of the acid is going to be nitric acid.
Let's Practice
Practice what you just learned by answering the question on each slide, then check your answer.
How can an acid be easily identified from its chemical formula?
Begins with H
Acids always contain hydrogen ion(s), H+, so their formulas can be easily identified because they will start with H. For example, some acid formulas include HCl, H2SO4, H3PO3, etcetera.
What is the anion present in hydrosulfuric acid?
Sulfide ion, S2−
An acid whose name begins with “hydro-” and ends with “-ic” contains a monatomic anion with the ending “-ide.” Therefore, hydrosulfuric acid contains the sulfide ion, S2−. Note that when writing the names of acids with the sulfide, sulfate, or sulfite ions, instead of just dropping the ending to make the root part of the name “sulf”, the root “sulfur” is used. Thus, this is “hydrosulfuric” acid, rather than “hydrosulfic” acid.
How many oxygen atoms are in the formula for nitrous acid?
Two oxygen atoms
An acid whose name ends with “-ous” contains a polyatomic anion with the ending “-ite.” Therefore, nitrous acid contains the nitrite ion, NO2−.
How many oxygen atoms are in the formula for carbonic acid?
Three oxygen atoms
An acid whose name ends with “-ic” contains a polyatomic anion with the ending “-ate.” Therefore, carbonic acid contains the carbonate ion, CO32−.
What is the anion present in phosphorous acid?
Phosphite ion, PO33−
An acid whose name ends with “-ous” contains a polyatomic anion with the ending “-ite.” Therefore, phosphorous acid contains the phosphite ion. Note that when writing the name of acids with the phosphide, phosphate, or phosphite ions, instead of just dropping the ending to make the root part of the word “phos,” the root “phosphor” is used.
