Formulas and Composition

From the name of a compound you can determine its formula and from that you can determine the formula mass of the compound (the molecular mass if it is a molecular material) and its composition in mass percent. The pages listed here give you information and examples of how to do this. Determining the formula of a compound from its name is crucial to each of the tasks remaining in this lesson.

Objective 7: From the name of a compound, determine the formula and formula mass

To determine the formula of a compound from its name, first consider what type of name it is. (More information on this is available near the beginning of this lesson in "Types of Names" section.) If it is a prefix name, use the prefixes to determine how many atoms of each element are contained in the formula.

If it is an ionic name, remember that the formula must be charge balanced. So even a simple-sounding name like "sodium nitride" requires you to remember that sodium has a +1 charge and nitride has a +3, so the charges must balance out with the formula Na₃N .

Once you have the formula, the formula mass is determined by adding up the atomic masses of each element, using each one as many times as is indicated by the subscripts in the formula.

Exercises

Determine the formula and formula mass for each of the following compounds:

a. water

b. magnesium chloride

c. dichlorine pentoxide

d. copper (I) oxide

e. carbon tetrabromide

f. sodium sulfate

h. barium nitride

Answers

a. water is H₂O, and its formula mass is 18.01 amu

b. magnesium chloride is MgCl₂ and its formula mass is 95.3 amu

c. dichlorine pentoxide is Cl₂O₅ and its formula mass is 151.0 amu

d. copper (I) oxide is Cu₂O, and its formula mass is 143.0 amu

e. carbon tetrabromide is CBr₄, and its formula mass is 331.6 amu

f. sodium sulfate is Na₂SO₄, and its formula mass is 142.1 amu

h. barium nitride is Ba₃N₂, and its formula mass is 439.9 amu

Objective 8: From the name of a compound, determine the composition (mass percent)

To determine the composition of a compound you need to expand on what you just did in the previous exercise. That involves figuring out the formula, then the formula mass and then the percentage of each one.

Exercises

What is the composition (in mass percent of each element) of the following compounds?

a. calcium nitrate

b. sodium sulfide

c. carbon dioxide

d. iron(III) chloride

e. iron(III) sulfide

f. aluminum oxide

g. ammonium carbonate

Answers

a. calcium nitrate - Ca(NO₃)₂

24.4% Ca

17.1% N

58.5% O

b. sodium sulfide - Na₂S

58.9% Na

41.1% S

c. carbon dioxide - CO₂

27.3% C

72.7% O

d. iron(III) chloride - FeCl₃

34.4% Fe

65.6% Cl

e. iron(III) sulfide - Fe₂S₃

53.7% Fe

46.3% S

f. aluminum oxide - Al₂O₃

52.9% Al

47.1% O

g. ammonium carbonate - (NH₄)₂CO₃

29.1% N

8.4% H

12.5% C

49.9% O