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Determining Formula and
Molar Masses
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Printed: November 16, 2014
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C HAPTER
Chapter 1. Determining Formula and Molar Masses
1
Determining Formula and
Molar Masses
Lesson Objectives
The student will:
• calculate the formula mass of a compound given its name and a periodic table.
Vocabulary
• formula mass
• molecular mass
Introduction
When atoms of one element chemically combine with atoms of another element, a compound is formed. Compounds
have names and formulas associated with them. The formula of a compound contains chemical symbols that tell us
what elements are in the compound. The subscripts in the formula tell us the ratios of the elements present. For
example, the formula MgCl2 tells us that this compound is composed of the elements magnesium and chlorine, which
combine in the ratio of two atoms of chlorine for each atom of magnesium. Using the formula of the compound
and the relative masses of elements from the periodic table, we can then calculate the relative formula mass for the
compound.
Formula Mass
The periodic table tells us the relative masses of all the elements. Looking at the squares for carbon and helium, we
can see that a carbon atom has about three times the mass of a helium atom. In this way, we can compare the relative
masses of any two atoms in the table. By looking at the chemical formulas and the relative atomic masses, we can
also compare the masses of different compounds. The formula mass of a compound is the sum of all the atomic
masses in the formula. The formula for water, for example, is H2 O. This formula tells us that water is composed of
hydrogen and oxygen and has a ratio of two hydrogen atoms for each oxygen atom. We can determine the formula
mass for water by adding up the atomic masses of its components.
Example:
What is the formula mass of H2 O?
Solution:
Element Atomic Mass Number of Atoms
per Formula
H
1.0 daltons
2
O
16.0 daltons
1
Product
2.0 daltons
16.0 daltons
18.0 daltons
The formula mass of H2 O = 18.0 daltons.
Example:
1
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What is the formula mass of Ca(NO3 )2 ?
Solution:
Element Atomic Mass Number of Atoms
per Formula
Ca
40.0 daltons
1
N
14.0 daltons
2
O
16.0 daltons
6
Product
40.0 daltons
28.0 daltons
96.0 daltons
164.0 daltons
The formula mass of Ca(NO3 )2 = 164.0 daltons.
These formula masses are in the same units as atomic masses and therefore are exactly comparable. For example,
the atomic mass of an oxygen atom is 16 daltons, the atomic mass of a fluorine atom is 19 daltons, and the formula
mass of a water molecule is 18 daltons. This means that a water molecule is slightly more massive than an oxygen
atom and slightly less massive than a fluorine atom.
Terminology
Since ionic compounds do not form individual molecules, the term formula mass is the only proper term to describe
this relative mass. In comparison, the formula mass for covalent compounds may also be called the molecular mass
because covalent compounds do form molecules. While it is important to understand this distinction, no professional
chemists have had their degrees in chemistry recalled when they referred to the “molecular mass” of NaCl due to a
slip of the tongue.
Lesson Summary
• The molecular mass of a molecule is found by adding the atomic masses of all the atoms in one molecule.
• Not all substances exist as molecules, so the term molecular mass is not used for all substances. The masses
of ionic compounds and empirical formulas are called formula mass.
Review Questions
1. Calculate the formula mass for each of the following.
a.
b.
c.
d.
e.
f.
g.
h.
K2 SO4
CuO
Mg3 (AsO4 )2
Ca3 (PO4 )2
Fe2 O3
Al(OH)3
(NH4 )2 S
C12 H22 O11
2. On average, how many times heavier are bromine atoms than neon atoms?
3. An unknown element, M, combines with oxygen to form a compound with a formula of MO2 . If 25.0 grams
of the unknown element combines with 4.50 grams of oxygen, what is the atomic mass of M?
Answers:
a. 174
b. 79.5
2
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Chapter 1. Determining Formula and Molar Masses
c. 351
d. 310
e. 165
f. 78
g. 68
h. 342
2. 4 times larger
3. 177.8 g/mol
3