## (8) Formula and Formula Weight

### Obj. 8. From the name of a *compound*, determine the
formula and formula weight.

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 a **Stock name**, several steps are needed. Use the Roman numeral
to determine the charge (or oxidation state) of the first element. From the second part of
the name, determine whether it is a simple anion or a polyatomic ion. If it is a simple
anion, determine its charge (or oxidation state) from its position on the periodic table.
If it is a polyatomic ion, determine its charge from memory. Use the charges on the ions
to determine the formula of the compound.

If it is a **Latin name**, several steps are needed. Use the *-ous*
or *-ic* suffix to determine whether the charge (or oxidation state) of the first
element is the lower or higher of its common charges. You might remember the common
charges of the element, or you may have to look them up. From the second part of the name,
determine whether it is a simple anion or a polyatomic ion. If it is a simple anion,
determine its charge (or oxidation state) from its position on the periodic table. If it
is a polyatomic ion, determine its charge from memory. Use the charges on the ions to
determine the formula of the compound.

If it is a **simple name**, first determine whether the compound is ionic
or covalent. If it is ionic, determine the charges on the ions from their postitions on
the periodic table. Then use the charges on the ions to determine the formula of the
compound. If the compound is covalent, use the periodic table to determine how many
electrons each element needs to gain. Reverse (and simplify) the "need ratio" to
get the "atom ratio" and the formula of the compound.

If it is a **common name** you will need to determine the formula from
memory (or by looking up the compound).

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

### Exercises

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

a. ammonia

b. magnesium chloride

c. chlorine(V) oxide

d. cuprous oxide

e. carbon tetrabromide

f. sodium sulfate

h. barium nitride

### Worked-Out Examples (a,b)

(a) Ammonia, you should have learned, is NH_{3}. Even if all you remember is
that ammonia contains nitrogen and hydrogen, you should be able to figure out the formula.
Because they are both nonmetals you know that each wants to gain electrons. Nitrogen wants
to gain three electrons; hydrogen wants to gain one electron; therefore, it will take
three hydrogens to satisfy the needs of the one nitrogen atom. So the formula will be NH_{3}.

Knowing that the formula is NH_{3}, you can come up with the formula weight by
adding up the atomic weights. There are different degrees of precision to which you can do
this. For this course you should, as a general rule, use one decimal place in the atomic
weights. Nitrogen has an atomic weight of 14.0; hydrogen has an atomic weight of 1.0; so
three of them would be 3.0. That gives the sum of 17.0. So the formula weight of ammonia
is 17.0, in either grams per mole or amu per molecule.

(b) Magnesium chloride is an ionic compound because it has a metal, magnesium, and a
nonmetal, chlorine. Magnesium will lose two electrons and form a +2 charge. Chlorine will
gain one electron to form a chloride ion with a -1 charge. The formula for the compound is
MgCl_{2}.

To get the formula weight, find the atomic weights and add them together taking the
subscripts into account. Magnesium is 24.3; chlorine is 35.5; so two would be 71.0. The
total gives 95.3 as the formula weight.

### Answers to Exercises

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

a. ammonia is NH_{3}, and its formula weight is 17.0

b. magnesium chloride is MgCl_{2} and its formula weight is 95.3

c. chlorine(V) oxide is Cl_{2}O_{5} and its formula weight is 151.0

d. cuprous oxide is Cu_{2}O, and its formula weight is 143.0

e. carbon tetrabromide is CBr_{4}, and its formula weight is 331.6

f. sodium sulfate is Na_{2}SO_{4}, and its formula weight is 142.1

h. barium nitride is Ba_{3}N_{2}, and its formula weight is 439.9

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