Chapter 7
Section 3 Using Chemical Formulas
Preview
• Lesson Starter
• Objectives
• Formula Masses
• Molar Masses
• Molar Mass as a Conversion Factor
• Percentage Composition
Chapter 7
Section 3 Using Chemical Formulas
Lesson Starter
• The chemical formula for water is H2O.
• How many atoms of hydrogen and oxygen are there
in one water molecule?
• How might you calculate the mass of a water
molecule, given the atomic masses of hydrogen and
oxygen?
• In this section, you will learn how to carry out these
and other calculations for any compound.
Chapter 7
Section 3 Using Chemical Formulas
Objectives
• Calculate the formula mass or molar mass of any given
compound.
• Use molar mass to convert between mass in grams and
amount in moles of a chemical compound.
• Calculate the number of molecules, formula units, or
ions in a given molar amount of a chemical compound.
• Calculate the percentage composition of a given
chemical compound.
Chapter 7
Section 3 Using Chemical Formulas
• A chemical formula indicates:
• the elements present in a compound
• the relative number of atoms or ions of each
element present in a compound
• Chemical formulas also allow chemists to calculate a
number of other characteristic values for a compound:
• formula mass
• molar mass
• percentage composition
Chapter 7
Section 3 Using Chemical Formulas
Formula Masses
• The formula mass of any molecule, formula unit, or
ion is the sum of the average atomic masses of all
atoms represented in its formula.
• example: formula mass of water, H2O
average atomic mass of H: 1.01 amu
average atomic mass of O: 16.00 amu
1
.
0
1
a
m
u
2
H
a
t
o
m
s


2
.
0
2
a
m
u
H
a
t
o
m
1
6
.
0
0
a
m
u
1
O
a
t
o
m


1
6
.
0
0
a
m
u
O
a
t
o
m
average mass of H2O molecule: 18.02 amu
Chapter 7
Section 3 Using Chemical Formulas
Formula Masses
• The mass of a water molecule can be referred to as a
molecular mass.
• The mass of one formula unit of an ionic compound,
such as NaCl, is not a molecular mass.
• The mass of any unit represented by a chemical
formula (H2O, NaCl) can be referred to as the formula
mass.
Chapter 7
Section 3 Using Chemical Formulas
Formula Masses, continued
Sample Problem F
Find the formula mass of potassium chlorate, KClO3.
Chapter 7
Section 3 Using Chemical Formulas
Formula Masses, continued
Sample Problem F Solution
The mass of a formula unit of KClO3 is found by
adding the masses of one K atom, one Cl atom, and
three O atoms.
Atomic masses can be found in the periodic table in
the back of your book.
In your calculations, round each atomic mass to two
decimal places.
Chapter 7
Section 3 Using Chemical Formulas
Formula Masses, continued
Sample Problem F Solution, continued
3
9
.
1
0
a
m
u
1
K
a
t
o
m


3
9
.
1
0
a
m
u
K
a
t
o
m
3
5
.
4
5
a
m
u
1
C
l
a
t
o
m


3
5
.
4
5
a
m
u
C
l
a
t
o
m
1
6
.
0
0
a
m
u
3
O
a
t
o
m
s


4
8
.
0
0
a
m
u
O
a
t
o
m
formula mass of KClO3 = 122.55 amu
Chapter 7
Section 3 Using Chemical Formulas
Molar Masses
• The molar mass of a substance is equal to the mass
in grams of one mole, or approximately 6.022 × 1023
particles, of the substance.
• example: the molar mass of pure calcium, Ca, is 40.08 g/mol
because one mole of calcium atoms has a mass of 40.08 g.
• The molar mass of a compound is calculated by
adding the masses of the elements present in a mole
of the molecules or formula units that make up the
compound.
Chapter 7
Section 3 Using Chemical Formulas
Molar Masses, continued
• One mole of water molecules contains exactly two
moles of H atoms and one mole of O atoms. The
molar mass of water is calculated as follows.
1
.
0
1
g
H
2
m
o
lH


2
.
0
2
g
H
m
o
lH
1
6
.
0
0
g
O
1
m
o
lO


1
6
.
0
0
g
O
m
o
lO
molar mass of H2O molecule: 18.02 g/mol
• A compound’s molar mass is numerically equal to its
formula mass.
Chapter 7
Section 3 Using Chemical Formulas
Calculating Molar Masses for Ionic Compounds
Chapter 7
Section 3 Using Chemical Formulas
Molar Masses, continued
Sample Problem G
What is the molar mass of barium nitrate, Ba(NO3)2?
Chapter 7
Section 3 Using Chemical Formulas
Molar Masses, continued
Sample Problem G Solution
One mole of barium nitrate, contains one mole of Ba, two moles of
N (1 × 2), and six moles of O (3 × 2).
1
3
7
.
3
3
g
H
1
m
o
l
B
a


1
3
7
.
3
3
g
B
a
m
o
l
B
a
1
4
.
0
1
g
2
m
o
lN


2
8
.
0
2
g
N
m
o
lN
1
6
.
0
0
g
O
6
m
o
lO


9
6
.
0
0
g
O
m
o
lO
molar mass of Ba(NO3)2 = 261.35 g/mol
Chapter 7
Section 3 Using Chemical Formulas
Molar Mass as a Conversion Factor
• The molar mass of a compound can be used as a
conversion factor to relate an amount in moles to a
mass in grams for a given substance.
• To convert moles to grams, multiply the amount in
moles by the molar mass:
Amount in moles × molar mass (g/mol)
= mass in grams
Chapter 7
Section 3 Using Chemical Formulas
Mole-Mass Calculations
Chapter 7
Section 3 Using Chemical Formulas
Molar Mass as a Conversion Factor, continued
Sample Problem H
What is the mass in grams of 2.50 mol of oxygen gas?
Chapter 7
Section 3 Using Chemical Formulas
Molar Mass as a Conversion Factor, continued
Sample Problem H Solution
Given: 2.50 mol O2
Unknown: mass of O2 in grams
Solution:
moles O2
grams O2
amount of O2 (mol) × molar mass of O2 (g/mol) =
mass of O2 (g)
Chapter 7
Section 3 Using Chemical Formulas
Molar Mass as a Conversion Factor, continued
Sample Problem H Solution, continued
Calculate the molar mass of O2.
1
6
.
0
0
g
O
2
m
o
lO


3
2
.
0
0
g
m
o
lO
Use the molar mass of O2 to convert moles to mass.
3
2
.
0
0
g
O
2
2
.
5
0
m
o
l
O


8
0
.
0
g
O
2
2
m
o
l
O
2
Chapter 7
Section 3 Using Chemical Formulas
Converting Between Amount in Moles and
Number of Particles
Chapter 7
Section 3 Using Chemical Formulas
Molar Mass as a Conversion Factor, continued
Sample Problem I
Ibuprofen, C13H18O2, is the active ingredient in many
nonprescription pain relievers. Its molar mass is
206.31 g/mol.
a. If the tablets in a bottle contain a total of 33 g of
ibuprofen, how many moles of ibuprofen are in the
bottle?
b. How many molecules of ibuprofen are in the bottle?
c. What is the total mass in grams of carbon in 33 g
of ibuprofen?
Chapter 7
Section 3 Using Chemical Formulas
Molar Mass as a Conversion Factor, continued
Sample Problem I Solution
Given:
33 g of C13H18O2
molar mass 206.31 g/mol
Unknown:
a. moles C13H18O2
b. molecules C13H18O2
c. total mass of C
Solution:
a. grams
moles
1
m
o
l
C
H
O
1
3
1
8
2
g
C
H
O


m
o
l
C
H
O
1
3
1
8
2
1
3
1
8
2
2
0
6
.
3
1
g
C
H
O
1
3
1
8
2
Chapter 7
Section 3 Using Chemical Formulas
Molar Mass as a Conversion Factor, continued
Sample Problem I Solution, continued
b. moles
molecules
6
.
0
2
2

1
0
m
o
l
e
c
u
l
e
s
m
o
l
C
H
O


m
o
l
e
c
u
l
e
s
C
H
O
1
3
1
8
2
1
3
1
8
2
m
o
l
2
3
c. moles C13H18O2
moles C
grams C
1
3
m
o
l
C
1
2
.
0
1
g
C
m
o
l
C
H
O



g
C
1
3
1
8
2
m
o
l
C
H
O
m
o
l
C
1
3
1
8
2
Chapter 7
Section 3 Using Chemical Formulas
Molar Mass as a Conversion Factor, continued
Sample Problem I Solution, continued
a.
1
m
o
l
C
H
O
1
3
1
8
2
3
3
g
C
H
O


0
.
1
6
m
o
l
C
H
O
1
3
1
8
2
1
3
1
8
2
2
0
6
.
3
1
g
C
H
O
1
3
1
8
2
b.
6
.
0
2
2

1
0m
o
l
e
c
u
l
e
s
0
.
1
6
m
o
lC
H
O


1
31
82
m
o
l
2
3
9
.
6

1
0m
o
l
e
c
u
l
e
s
C
H
O
1
31
82
2
2
c.
1
3
m
o
l
C
1
2
.
0
1
g
C
0
.
1
6
m
o
l
C
H
O



2
5
g
C
1
3
1
8
2
m
o
l
C
H
O
m
o
l
C
1
3
1
8
2
Chapter 7
Section 3 Using Chemical Formulas
Percentage Composition
• It is often useful to know the percentage by mass of
a particular element in a chemical compound.
• To find the mass percentage of an element in a
compound, the following equation can be used.
m
a
s
s
o
fe
l
e
m
e
n
t
i
n
s
a
m
p
l
e
o
fc
o
m
p
o
u
n
d

1
0
0

m
a
s
s
o
fs
a
m
p
l
e
o
fc
o
m
p
o
u
n
d
%
e
l
e
m
e
n
t
i
n
c
o
m
p
o
u
n
d
• The mass percentage of an element in a compound
is the same regardless of the sample’s size.
Chapter 7
Section 3 Using Chemical Formulas
Percentage Composition, continued
• The percentage of an element in a compound can be
calculated by determining how many grams of the
element are present in one mole of the compound.
m
a
s
s
o
fe
l
e
m
e
n
ti
n
1
m
o
lo
fc
o
m
p
o
u
n
d

1
0
0

m
o
l
a
r
m
a
s
s
o
fc
o
m
p
o
u
n
d
%
e
l
e
m
e
n
ti
n
c
o
m
p
o
u
n
d
• The percentage by mass of each element in a
compound is known as the percentage
composition of the compound.
Chapter 7
Section 3 Using Chemical Formulas
Percentage Composition of Iron Oxides
Chapter 7
Section 3 Using Chemical Formulas
Percentage Composition Calculations
Chapter 7
Section 3 Using Chemical Formulas
Percentage Composition, continued
Sample Problem J
Find the percentage composition of copper(I) sulfide,
Cu2S.
Chapter 7
Section 3 Using Chemical Formulas
Percentage Composition, continued
Sample Problem J Solution
Given: formula, Cu2S
Unknown: percentage composition of Cu2S
Solution:
formula
molar mass
mass percentage
of each element
Chapter 7
Section 3 Using Chemical Formulas
Percentage Composition, continued
Sample Problem J Solution, continued
6
3
.
5
5
g
C
u
2
m
o
l
C
u


1
2
7
.
1
g
C
u
m
o
l
C
u
3
2
.
0
7
g
S
1
m
o
lS


3
2
.
0
7
g
S
m
o
lS
Molar mass of Cu2S = 159.2 g
Chapter 7
Section 3 Using Chemical Formulas
Percentage Composition, continued
Sample Problem J Solution, continued
1
2
7
.
1
g
C
u

1
0
0

7
9
.
8
5
%
C
u
1
5
9
.
2
g
C
u
S
2
3
2
.
0
7
g
S

1
0
0

2
0
.
1
5
%
S
1
5
9
.
2
g
C
u
S
2