Introductory Chemistry: A Foundation - Zumdahl S.S., DeCoste D.J. 2019

Writing Formulas from Names


· To learn to write the formula of a compound, given its name.

So far we have started with the chemical formula of a compound and decided on its systematic name. Being able to reverse the process is also important. Often a laboratory procedure describes a compound by name, but the label on the bottle in the lab shows only the formula of the chemical it contains. It is essential that you are able to get the formula of a compound from its name. In fact, you already know enough about compounds to do this. For example, given the name calcium hydroxide, you can write the formula as because you know that calcium forms only ions and that, because hydroxide is , two of these anions are required to give a neutral compound. Similarly, the name iron(II) oxide implies the formula , because the Roman numeral II indicates the presence of the cation and the oxide ion is .

Critical Thinking

· In this chapter, you have learned a systematic way to name chemical compounds. What if all compounds had only common names? What problems would this cause?

We emphasize at this point that it is essential to learn the name, composition, and charge of each of the common polyatomic anions (and the cation). If you do not recognize these ions by formula and by name, you will not be able to write the compound’s name given its formula or the compound’s formula given its name. You must also learn the names of the common acids.

Interactive Example 5.9. Writing Formulas from Names

Give the formula for each of the following compounds.

a. potassium hydroxide

b. sodium carbonate

c. nitric acid

d. cobalt(III) nitrate

e. calcium chloride

f. lead(IV) oxide

g. dinitrogen pentoxide

h. ammonium perchlorate






potassium hydroxide

Contains and .


sodium carbonate

We need two to balance .


nitric acid

Common strong acid; memorize.


cobalt(III) nitrate

Cobalt(III) means ; we need three to balance .


calcium chloride

We need two to balance ; (Group 2) always forms .


lead(IV) oxide

Lead(IV) means ; we need two to balance .


dinitrogen pentoxide

Di- means two; pent(a)- means five.


ammonium perchlorate

Contains and .

Self-Check: Exercise 5.8

· Write the formula for each of the following compounds.

a. ammonium sulfate

b. vanadium(V) fluoride

c. disulfur dichloride

d. rubidium peroxide

e. aluminum oxide

See Problems 5.41, 5.42, 5.43, 5.44, 5.45, and 5.46.