Chemistry Reference
In-Depth Information
Figure 7.1
Some Conversions
Involving Moles
Number of
formula units
Avogadro's
number
Number
of moles
Formula
mass
Mass
in grams
EXAMPLE 7.6
Knowing that the formula mass of ethyl alcohol,
C
2
H
6
O,
is 46.0 amu, show that
C
2
H
6
O
has a molar mass equal to 46.0 g/mol.
Solution
10
23
formula units
1 mol
46.0 amu
1 formula unit
a
6.02
1.00 g
46.0 g
1 mol
b
a
b
10
23
amu
6.02
Avogadro's number
From Example 7.5
Practice Problem 7.6
The formula mass of
H
3
PO
4
is 98.0 amu. What
is its molar mass (in grams per mole)?
We can use Avogadro's
number
as a conversion factor to convert moles to
numbers
of formula units, and vice versa. We can use the molar
mass
to con-
vert moles to
masses,
and vice versa (Figure 7.1).
EXAMPLE 7.7
(a)
Calculate the number of molecules in 0.445 mol of
C
4
H
8
.
(b)
Calculate the mass of 0.445 mol of
C
4
H
8
.
Solution
10
23
molecules C
4
H
8
1 mol C
4
H
8
a
6.02
(a)
0.445 mol
C
4
H
8
b
Moles of
C
4
H
8
Avogadro's
number
Molecules
of C
4
H
8
10
23
molecules
2.68
C
4
H
8
(b)
The molar mass of
C
4
H
8
is
4(12.01 g)
8(1.008 g)
56.10
4
g.
a
56.10
4
g C
4
H
8
1 mol C
4
H
8
Moles of
C
4
H
8
Molar
mass
Mass of
C
4
H
8
0.445 mol C
4
H
8
b
25.0 g C
4
H
8
Practice Problem 7.7
(a)
Calculate the number of moles of
C
5
H
12
in 41.9 g of
C
5
H
12
.
(b)
Calculate the number of moles of
C
5
H
12
in a sample containing
10
23
7.15
C
5
H
12
molecules.
EXAMPLE 7.8
Calculate the number of molecules of
SO
2
in 41.31 g of SO
2
.
