Chemistry Reference
In-Depth Information
must be used, they developed other ways to express concentrations.
Molality
is
the number of moles of solute per kilogram of solvent:
Molality is the number of moles
of solute per kilogram of solvent.
moles of solute
kilogram of solvent
Molality
Molality is symbolized
m.
The unit of molality is
molal,
and the symbol for molal
is m. The great similarities in name and meaning between molarity and molality
can be confusing. However, the concepts differ in two ways: For molality, (1)
mass
is used, not volume; and (2) the
solvent
is measured, not the solution. Be sure to
use M (capital) to represent molar, and m (lowercase) to represent molal.
EXAMPLE 15.7
Calculate the molality of a solution prepared by dissolving 29.4 g of NaCl in
212.7 g of water.
Solution
The number of moles of solute and the number of kilograms of solvent are
calculated first:
a
1 mol NaCl
58.5 g NaCl
29.4 g NaCl
b
0.502
6
mol NaCl
1 kg H
2
O
1000 g H
2
O
212.7 g H
2
O
a
b
0.2127 kg H
2
O
The number of moles of NaCl is then divided by the number of kilograms of
to obtain the molality:
H
2
O
0.502
6
mol NaCl
0.2127 kg H
2
O
m
2.36 m
Practice Problem 15.7
Calculate the molality of a solution prepared
by dissolving 28.2 g of
C
3
H
7
OH
in 207 mL of water (density
1.00 g/mL).
EXAMPLE 15.8
The solubility of
MgSO
4
in water at
0°C
is 26 g per 100.0 g of water. Express
this solubility as a molality.
Solution
26.0 g MgSO
4
100.0 g H
2
O
a
1 mol MgSO
4
120.4 g MgSO
4
a
1000 g H
2
O
1 kg H
2
O
2.16 mol MgSO
4
1 kg H
2
O
b
b
2.16 m MgSO
4
Many problems involving molality are similar to those involving molarity.
