Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
148 Chemistry: Matter and Change
Solving Problems: A Chemistry Handbook
SOLVING PROBLEMS:
A CHEMISTRY HANDBOOK
CHAPTER 14
colligative properties. One colligative property is vapor pressure
lowering. The vapor pressure of a solution containing a nonvolatile
solute is lower than the vapor pressure of the pure solvent. This is
because in the solution there are fewer solvent molecules at the surface from which evaporation takes place. Thus, there are fewer
solvent molecules escaping and less vapor produced, thereby lowering the vapor pressure. The greater the number of solute particles in
the solvent, the lower the vapor pressure.
The effects of solutes on colligative properties depend upon the
actual concentration of solute particles. For nonelectrolytes, the
solute particle concentration is the same as the solute concentration.
This is because nonelectrolytes, such as sucrose, do not ionize in
solution. Electrolytes are substances that ionize or that dissociate into
ions. Thus, electrolytes produce particle concentrations higher than
those of the original substance. For example, sodium chloride, NaCl,
dissociates almost completely into separate sodium ions and chloride
ions, so a 1m NaCl solution is actually nearly 2m in particles.
Two other colligative properties are the raising of a solution’s
boiling point and the reduction of its freezing point. The temperature
difference between the boiling point of a solution and the boiling
point of its pure solvent is called boiling point elevation. The
greater the number of solute particles in the solution, the greater is
the boiling point elevation. Boiling point elevation is symbolized
⌬T b and is related to the solution’s molality as follows
⌬T b ϭ K b m
where m is the molality and K b , the molal boiling point elevation
constant, has a value that depends on the particular solvent. For
water, K b is equal to 0.512°C/m.
The temperature difference between the freezing point of a solution and the freezing point of its pure solvent is called freezing
point depression. The freezing point of a solution is always lower
than that of a pure solvent. Freezing point depression is symbolized
⌬T f and is related to the solution’s molality as follows
⌬T f ϭ K f m
where m is the molality and K f , the molal freezing point depression
constant, has a value that depends on the particular solvent. For
water, K f is equal to 1.86°C/m.
148 Chemistry: Matter and Change
Solving Problems: A Chemistry Handbook
SOLVING PROBLEMS:
A CHEMISTRY HANDBOOK
CHAPTER 14
colligative properties. One colligative property is vapor pressure
lowering. The vapor pressure of a solution containing a nonvolatile
solute is lower than the vapor pressure of the pure solvent. This is
because in the solution there are fewer solvent molecules at the surface from which evaporation takes place. Thus, there are fewer
solvent molecules escaping and less vapor produced, thereby lowering the vapor pressure. The greater the number of solute particles in
the solvent, the lower the vapor pressure.
The effects of solutes on colligative properties depend upon the
actual concentration of solute particles. For nonelectrolytes, the
solute particle concentration is the same as the solute concentration.
This is because nonelectrolytes, such as sucrose, do not ionize in
solution. Electrolytes are substances that ionize or that dissociate into
ions. Thus, electrolytes produce particle concentrations higher than
those of the original substance. For example, sodium chloride, NaCl,
dissociates almost completely into separate sodium ions and chloride
ions, so a 1m NaCl solution is actually nearly 2m in particles.
Two other colligative properties are the raising of a solution’s
boiling point and the reduction of its freezing point. The temperature
difference between the boiling point of a solution and the boiling
point of its pure solvent is called boiling point elevation. The
greater the number of solute particles in the solution, the greater is
the boiling point elevation. Boiling point elevation is symbolized
⌬T b and is related to the solution’s molality as follows
⌬T b ϭ K b m
where m is the molality and K b , the molal boiling point elevation
constant, has a value that depends on the particular solvent. For
water, K b is equal to 0.512°C/m.
The temperature difference between the freezing point of a solution and the freezing point of its pure solvent is called freezing
point depression. The freezing point of a solution is always lower
than that of a pure solvent. Freezing point depression is symbolized
⌬T f and is related to the solution’s molality as follows
⌬T f ϭ K f m
where m is the molality and K f , the molal freezing point depression
constant, has a value that depends on the particular solvent. For
water, K f is equal to 1.86°C/m.