Effects of solute on
solvent
12.5-12.6
Homework Check
 39-57
4
sheets
Vapor Pressure
 Define:
 The
pressure of the vapor created by
the solution evaporating.
–Must be volatile solution
Evaporates easily
Effect of Nonvolatile Solute
 Causes
vapor pressure to decrease
–Concentration volatile solvent
decrease
–Less ability to evaporate
Partial Pressure
Raoult’s law
 Colligative
property
 Dependent on mole fraction
Psoln = PopureXpure
Vapor Pressure Lowering
 Subtract
– Pressurepure
 Vapor
solvent
- Pressuresolution
Pressure Lowering Formula
DP = PopureXsolute
 EX.
Calculate the vapor-pressure
lowering of water when 5.67 g of
glucose is dissolved in 25.2 g of
water at 25oC. The vapor pressure
of water at 25oC is 23.8 mmHg.
What is the vapor pressure of the
solution?

Naphthalene, C10H8 is used to make mothballs.
Suppose a solution is made by dissolving 0.515
g of naphthalene in60.8 g of chloroform, CHCl3.
Calculate the vapor-pressure lowering of
chloroform at 20oC from the naphthalene. The
vapor pressure of chloroform at 20oC is 156
mmHg. Naphthalene can be assumed to be
nonvolatile compared with chloroform. What is
the vapor pressure of the solution?
 You
need to boil a water-based
solution at a temperature lower than
100oC. What kind of liquid could you
add to the water to make this
happen?
Boiling Point Elevation
 The
addition of a nonvolatile solute
to a liquid reduces its vapor
pressure; therefore, the temperature
must be increased in order to
achieve a set vapor pressure in order
to boil. Hence Bp elevation.
Boiling point elevation,
 DTb
is a colligative property of a
solution equal to the boiling point of
the solution minus the boiling point
of the pure solvent. (p 523)
DTb = KbCm
 Kb = boiling point constant listed on
p523
 Cm = molal concentration
Freezing point depression
 DTf,
is a colligative property of a
solution equal to the freezing point of
the pure solvent minus the freezing
point of the solution.
DTf = KfCm
(Kf = Fp constant p 523)
 EX.
An aqueous solution (dissolved in
H2O) is 0.0222 m glucose. What are
the boiling point and the freezing
point of this solution?
Calculation Molar Mass
 From
the freezing point lowering,
you can calculate the molal
concentration and from the molality,
you can obtain the molecular weight.
 Find moles solute (gsolvent X molality)
 gsolute / moles = MM
 EX.
A solution is prepared by
dissolving 0.131 g of a substance in
25.4 g of water. The molality of the
solution is determined by freezing
point depression to be 0.056 m.
What is the molecular weight of the
substance?
 EX.
Camphor melts at 179.5oC. Its
freezing point constant is 40oC/m. A
1.07 mg sample of a compound was
dissolved in 78.1 mg of camphor.
The solution melted at 176.0oC.
What is the molecular weight of the
compound? If the empirical formula
of the compound is CH, what is the
molecular formula?
Homework
Page: 545
Q: 59-67