Page 113 - Theory and Problems of BEGINNING CHEMISTRY

CHAPTER 7

Formula

Calculations

7.1. INTRODUCTION
Atoms and their symbols were introduced in Chaps. 1 and 3. In Sec 5.2, the representation of compounds
by their formulas was developed. The procedure for writing formulas from knowledge of the elements involved
or from names was presented in Chaps. 5 and 6. The formula for a compound contains much information of use
to the chemist, including the number of atoms of each element in a formula unit of a compound. Since atoms
are so tiny, we will learn to use large groups of atoms—moles of atoms—to ease our calculations (Sec. 7.4). We
will learn to calculate the percent by mass of each element in the compound (Sec. 7.5). We will learn how to
calculate the simplest formula from percent composition data (Sec. 7.6) and molecular formulas from simplest
formulas and molecular masses (Sec. 7.7).

7.2. MOLECULES AND FORMULA UNITS
Some elements combine by covalent bonding (Chap. 5) into units called molecules. Other elements combine
by gaining or losing electrons to form ions, which are charged atoms (or groups of atoms). The ions are attracted
to one another, a type of bonding called ionic bonding (Chap. 5), and can form combinations containing millions
or more of ions of each element. To identify these ionic compounds, the simplest formula is generally used. We
can tell that a compound is ionic if it contains at least one metal atom or the NH 4 group. If not, it is most likely
bonded in molecules (Chap. 5).
Formulas for ionic compounds represent one formula unit. However, molecules, too, have formulas and
thus formula units. Even uncombined atoms of an element have formulas. Thus, formula units may refer to
uncombined atoms, molecules, or atoms combined in ionic compounds; one formula unit may be
One atom of uncombined element, for example, Pt
One molecule of a covalently bonded compound, for example, H 2 O

One simple unit of an ionic compound, for example, NaCl or (NH 4 ) 2 SO 4
EXAMPLE 7.1. What are the formula units of the element copper, the compound dinitrogen trioxide, and the compound
sodium bromide?
Ans. The formula unit of copper is Cu, one atom of copper. The formula unit of dinitrogen trioxide is N 2 O 3 , one molecule
of dinitrogen trioxide. The formula unit of sodium bromide is NaBr.
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