Chemistry Reference
In-Depth Information
Percent by mass is a measure of concentration, de-
fined as 100.0% times the mass of a component of the so-
lution divided by the total mass of the solution. It is a
temperature-independent concentration unit. It is often
used for solutions in applied chemistry situations, such as
medical laboratories and hospitals (Section 15.3).
Molality
(m)
is a temperature-independent measure
of concentration, defined as the number of moles of
solute per kilogram of solvent. It differs from molarity
(M)
in that it is based on a
mass
of
solvent,
rather than a
volume of solution. Like molarity, molality can be used
as a factor to solve problems (Section 15.4). Molality is
also used in problems involving freezing-point depres-
sion and boiling-point elevation.
A third temperature-independent measure of con-
centration is mole fraction. It does not distinguish be-
tween solute and solvent. The mole fraction is defined
as the number of moles of one component, say compo-
nent A, divided by the total number of moles in the
solution:
lower than 1 in a solution of A, must be lower than
a vapor-pressure lowering has resulted because of the
presence of the solute.
The freezing point of a solvent is lowered by the
presence of a solute, and the boiling point of a solution
is raised by the presence of a nonvolatile solute. The
freezing-point depression and the boiling-point elevation
are both directly proportional to the molality of the solute
particles. Determining the molality therefore allows us to
calculate the freezing point or boiling point of a solution;
conversely, determining the freezing point or boiling
point allows us to calculate the molality. With the molal-
ity and other data, we are able to calculate the number of
moles and the molar mass.
Osmotic pressure is another property due to dis-
solved substances. The presence of solute particles
lowers the ability of solvent molecules to pass through
a semipermeable membrane. Osmotic pressure is very
important in biological systems, and an application of
the theory behind osmotic pressure allows for the
purification of seawater. The osmotic pressure of a so-
lution,
P
A
P
A
;
number of moles of A
total number of moles
, is proportional to the molarity (the number of
moles per liter):
X
A
(Section 15.5).
The presence of a solute affects some of the physical
properties of a solution, but the identity of the solute
makes little difference in the colligative properties. Vapor-
pressure lowering, freezing-point depression, boiling-
point elevation, and osmotic pressure are four such
properties.
The vapor pressure of a solvent in the presence of a
nonvolatile solute is given by Raoult's law:
V
nRT
or
a
n
V
b
RT
MRT
where
R
is the ideal gas law constant. Osmotic pressure
measurement is a very sensitive means of determining
the number of moles of solute in a given volume of so-
lution and, together with other data, the molar mass
(Section 15.6).
P
A
X
A
P
A
where
P
A
is the vapor pressure of the solvent in the solu-
tion,
X
A
is the mole fraction of A, and
P
A
is the vapor
pressure of the solvent when pure. Because
X
A
must be
Items for Special Attention
Be careful to distinguish between the mass of solution
and the mass of solvent in percent by mass and molality
problems. Percent by mass uses the former, and molality
the latter.
Be sure to remember the difference between
vapor pres-
sure
and
vapor-pressure lowering,
between
freezing point
and
freezing-point depression,
and between
boiling point
and
boiling-point elevation.
■
■
Molarity and molality are very similar; be careful not to
confuse them. Also, be sure to use M to represent molar
and m to represent molal.
Vapor-pressure lowering calculations involve mole frac-
tion; freezing-point depression and boiling-point elevation
calculations use molality; and osmotic pressure is calcu-
lated with molarity.
■
■
Osmotic pressure is a colligative property.
■
