Chemistry Reference
In-Depth Information
EXAMPLE 15.13
Show that the sum of the mole fractions of the components of the solution of
Example 15.12 is equal to 1.
Solution
The mole fraction of the ethyl alcohol is
2.19 mol
1.46 mol
X
C
2
H
5
OH
2.19 mol
0.600
Adding this mole fraction to that of methyl alcohol (obtained in Example 15.12)
gives
0.400
0.600
1.000
Practice Problem 15.13
Which, if either, of the two substances in
Example 15.12 is regarded as the solute?
EXAMPLE 15.14
Calculate the molality of a solution of methyl alcohol,
CH
3
OH,
in water in which
the alcohol has a mole fraction of 0.133.
Solution
Because both concentration units are intensive, we can choose any quantity of
solution we wish (or we can solve using algebraic unknowns). It is generally
easiest to assume a quantity in which one or more numbers of moles is easiest
to determine; in this case we will choose 1.000 mol total. Thus we have
0.133 mol and 0.867 mol water. We merely change the water to kilo-
grams to determine the molality.
Choose a quantity that makes at
least one number of moles easy
to calculate.
CH
3
OH
a
18.0 g H
2
O
1 mol H
2
O
0.867 mol H
2
O
b
15.6 g
0.0156 kg
0.133 mol CH
3
OH
0.0156 kg H
2
O
8.53 m
Practice Problem 15.14
Calculate the mole fractions of a 1.27 m
solution of methyl alcohol in water.
Snapshot Review
❒
The mole fraction of a substance in a solution is the number of moles
of the substance divided by the total number of moles in the solution.
ChemSkill Builder 15.2
A. Calculate the mole fraction of sucrose, (table sugar), if
1.16 mol is dissolved in 55.6 mol of water (1.00 kg).
C
12
H
22
O
11
