Chemistry Reference
In-Depth Information
atoms of another element or moles of entire formula units
(see Figure 7.2). We must consider the units (for example,
moles of atoms versus number of individual atoms versus
mass of the atoms) as well as the species (for example,
atoms of an element versus molecules of that element).
These differences are apparent in the questions: “How
many
atoms
of oxygen are in 2.00
mol
of oxygen
mole-
cules?
” and “What is the
mass
of the
oxygen atoms
in
water molecules?
”
Do not use the term “molecule” or “molecular mass” when
discussing ionic compounds, because they do not exist as
molecules.
■
The unit usually used for the mass of a small number of
atoms, molecules, or formula units is the amu, which is a
very small fraction of a gram (about The unit
ordinarily used for the mass of molar quantities of a sub-
stance is the gram.
■
10
24
g).
1.00
10
23
The percentages of the elements making up any compound
must total 100%. If the percentages of all but one element
are given, the percentage of that element can easily be
calculated.
■
The most important skill learned in this chapter may be the
ability to convert from grams to moles of a substance, and
vice versa.
■
Answers to Snapshot Reviews
7.1 A. (a) 58.71 amu, atomic mass
(b) 394.7 amu, molecular mass
(c) 408.5 amu, neither
All three are formula masses.
C.
a
6.02
10
23
molecules C
3
H
7
NO
2
1 mol C
3
H
7
NO
2
3.50 mol C
3
H
7
NO
2
b
2.11
10
24
molecules C
3
H
7
NO
2
a
12.01 amu
32.04 amu
7.2 A. (a)
%C
b
100%
37.48% C
a
1 mol C
12.01 g C
b
7.4 A.
87.73 g C
7.305 mol C
a
4.032 amu
32.04 amu
a
1 mol H
1.008 g H
%H
b
100%
12.58% H
12.27 g H
b
12.17 mol H
a
16.00 amu
32.04 amu
The mole ratio of H to C is
%O
b
100%
49.94% O
12.17/7.305
1.666/1
5/3.
The empirical formula is
7.5 A. The empirical formula mass is 14.0 amu and the
molecular mass is 126 amu, so there are (126 amu)/
(14.0 amu) empirical formula units per molecule.
The molecular formula is
B. The empirical formula for sulfur is S, with mass
32.06 amu. There are 256 amu/32.06 amu
C
3
H
5
.
a
7[12.01 amu]
227.14 amu
(b)
%C
b
100%
37.01% C
a
5[1.008 amu]
227.14 amu
9
%H
b
100%
2.219% H
C
9
H
18
.
a
3[14.01 amu]
227.14 amu
8
empir-
%N
b
100%
18.50% N
ical formula units per molecule.
S
8
is the molecular
formula.
a
6[16.00 amu]
227.14 amu
%O
b
100%
42.26% O
10
23
7.3 A. (a)
6.02
(b) 52.4 g
a
89.0 g C
3
H
7
NO
2
1 mol C
3
H
7
NO
2
B.
3.50 mol C
3
H
7
NO
2
b
312 g C
3
H
7
NO
2
Self-Tutorial Problems
7.1
What small integer should we multiply each of the fol-
lowing ratios by to get a whole-number ratio? What ratio
results in each case?
7.2
What small integer should we multiply each of the fol-
lowing ratios by to get a whole-number ratio? What ratio
results in each case?
1.50
1
2.25
1
3.33
1
2.50
1
1.33
1
1.40
1
(a)
(b)
(c)
(a)
(b)
(c)
3.67
1
2.75
1
1.20
1
3.25
1
2.167
1
1.75
1
(d)
(e)
(f)
(d)
(e)
(f)
