Chemistry Reference
In-Depth Information
The percent composition of a compound can be calculated in terms of molar
masses instead of formula masses.
EXAMPLE 7.11
Calculate the percent composition of nitroglycerine,
C
3
H
5
N
3
O
9
,
using molar
masses instead of formula masses.
Solution
The mass of a mole of
C
3
H
5
N
3
O
9
is
3(12.01 g)
5(1.008 g)
3(14.01 g)
9(16.00 g)
227.1 g
The percentage of each element is given by
3(12.01 g) C
227.1 g C
7
H
5
N
3
O
6
a
b
100.0%
15.87% C
5(1.008 g) H
227.1 g C
7
H
5
N
3
O
6
a
b
100.0%
2.219% H
3(14.01 g) N
227.1 g C
7
H
5
N
3
O
6
a
b
100.0%
18.51% N
9(16.00 g) O
227.1 g C
7
H
5
N
3
O
6
a
b
100.0%
63.41% O
Total
100.01%
Practice Problem 7.11
Calculate the percent composition of trinitro-
toluene, TNT, C
7
H
5
N
3
O
6
.
Snapshot Review
10
23
❒
The mole is Avogadro's number of formula units—equal to
units.
6.02
❒
A mole of a substance has a mass (in grams) equal to its formula
mass, called the
molar mass,
which can be used as a factor in solving
problems.
ChemSkill Builder 4.1
A. A mole of a substance with formula mass 52.4 amu has (a) how many
formula units? (b) what mass?
B. Calculate the mass of 3.50 mol of the amino acid alanine,
C. Calculate the number of molecules in 3.50 mol of alanine.
C
3
H
7
NO
2
.
7.4
Empirical Formulas
The
empirical formula
of a compound is the formula that gives the lowest
whole-number ratio of atoms of all the elements. For example, the empirical
formula of fruit sugar, fructose,
C
6
H
12
O
6
,
is
CH
2
O.
The simplest ratio of carbon
