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CHAPTER 21: COLLIGATIVE PROPERTIES
Colligative properties are physical properties of a solution which depend on the number of solute particles dissolved in the solvent.
Examples:
vapor pressure lowering
boiling point elevation
freezing point depression
The magnitude of the effect is directly proportional to the number of solute molecules or ions present, not the type of particle.
Electrolytes have a greater effect on colligative properties than nonelectrolytes since they produce ions when they dissolve. Remember that substances that release or produce ions when dissolved in water (ionic compounds) are called electrolytes.
Examples: NH4Cl
contains two ions ( NH4+ and Cl - )
Al2(SO4)3 contains 5 ions (2Al3+ and
3SO42-)
C6H12O6 contains one molecule
H2O contains one molecule
Therefore, NH4Cl will raise the boiling point of water more than an equal amount of C6H12O6.
Vapor Pressure Changes: According to Raoult’s
Law the vapor pressure of a solution varies directly as the mole fraction
of the solvent. The formula is:
vapor pressure = ( mole fraction ) ( vapor pressure)
(solution)
(solvent)
(solvent)
*mole fraction (solvent) =
moles of solvent
moles solvent+moles solute
*and mole = mass . Refer
to the sample problem on p. 523. (Merrill
Chemistry)
gfm
Generally, for nonvolatile solutes (these solutes
do not vaporize), as the number of solute particles increases, the mole fraction of the
solvent decreases. This results in a lowering of the vapor pressure.
Therefore, a solution that contains a nonvolatile solute always has a lower vapor pressure
than the pure solvent. Examples of nonvolatile solutes include sucrose and sodium
chloride.
For solutions containing volatile solutes, the vapor pressure of the solution equals the sum of the partial vapor pressures. Examples of volatile solutes include alcohol and gasoline.
(mole fraction x vapor pressure = partial vapor pressure)
*See the sample problem on p. 524.
*Use the table on p. 459 for water vapor pressures.
Boiling point (bp) : the temperature at which the vapor
pressure equals the atmospheric pressure.
When a nonvolatile solute is added to a liquid solvent, solute particles at the surface
lower the vapor pressure of the solvent, resulting in an elevation of the
boiling point.
The bp elevation is proportional to the molal concentration of the solute particles. The formula is: +Tbp = Kbp x m
+Tbp = change in the boiling point (bp solution - bp solvent)
m = molality of the solute (for ionic solutes, multiply the molality by the number of particles in the formula unit)
Kbp = molal boiling point constant for water (0.515°C)
To determine the new boiling point, add the value of +Tbp to the bp of the solvent. See the sample problem on page 530.
The freezing point (fp) is lowered when a solute is
added to a solvent. The formula is:
+Tfp = Kfp x m
+Tfp =
change in the freezing point
m = molality (for ionic
solutes, multiply the molality by the number of particles in the formula unit)
Kfp = molal freezing point constant for water (1.853°C)
To determine the new freezing point, substract the value of +Tfp from the fp of the solvent.
The molecular mass of a solute can be determined by changes in the fp and bp. See the sample problem on p. 532.
Step 1: Calculate the molality of the solution. m = +Tfp
Kfp
Step 2: Since m = moles
or = mass ,
gfm = mass .
kg
gfm
(m)
(kg)
kg
Sample Problem: When 36.0g of a nonvolatile, molecular substance is dissolved in 100.0g of water, the solution begins to freeze at -3.72° C. What is the molecular mass (gfm) of the solute?
+Tfp = 0°C - (- 3.72°C) = 3.72°C
m = +Tfp = 3.72°C = 2.01 m
Kfp 1.853°C
gfm = 36.0g = 179.10 g/mole
(2.01m)(.1kg)
Updated on August 3, 1999
