Lesson 10
Home Up (8) Formula and Formula Weight (9) Composition

 

(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 NH3. 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 NH3.

Knowing that the formula is NH3, 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 MgCl2.

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 NH3, and its formula weight is 17.0
b. magnesium chloride is MgCl2 and its formula weight is 95.3
c. chlorine(V) oxide is Cl2O5 and its formula weight is 151.0
d. cuprous oxide is Cu2O, and its formula weight is 143.0
e. carbon tetrabromide is CBr4, and its formula weight is 331.6
f. sodium sulfate is Na2SO4, and its formula weight is 142.1
h. barium nitride is Ba3N2, and its formula weight is 439.9

 

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