NAME_____________________________________________PERIOD_____DATE _____________________
CHEMISTRY Colligative Properties of Solutions
1. Name the four colligative properties.
boiling point elevation, freezing point depression, vapor pressure depression, osmotic pressure lowering
2. Explain why the vapor pressure of a solvent would be lowered by the presence of a nonvolatile solute.
In evaporation, solvent molecules leave from the surface of the liquid. In a solution, many solvent molecules on
the surface of the liquid are attracted to solute particles, which is a stronger attraction than their attraction to
other solvent molecules. This means that these molecules will require more energy to evaporate (sever their
attraction) which means that fewer of them at any given temperature will have enough energy. If fewer molecules
can evaporate, the vapor pressure will be decreased.
3. The vapor pressure of a 1 m NaCl solution equals that of a 1m KI solution. Explain why the vapor pressure of
these solutions is the same.
The change in a (colligative) property is based on the number of particles that the solute breaks into (indicated by
the “i” value). In the case of NaCl and KI, both compounds break into two particles for every formula unit (i =
2), and they have the same molarity (number of moles per kg of solvent). Therefore they will have the same
number of moles of ions, and the same effect on vapor pressure.
4. Why is a higher temperature required for boiling a solution containing a nonvolatile solute than for boiling the
pure solvent?
Boiling occurs when vapor pressure is equal to atmospheric pressure. At any temperature, vapor pressure of a
solution is lower than that of a solvent (see #2). Therefore at the “normal” boiling temperature, the vapor
pressure will not be high enough to cause boiling…rather the temperature will need to be raised more to get the
vapor pressure equal to atmospheric pressure.
5. What happens on the molecular level when a liquid freezes? What effect does a nonvolatile solute have on this
process?
Liquids freeze when the intermolecular forces between two solvent molecules are strong enough to lock them into
place and limit their movement. Because of the attraction between solvent and solute molecules, in a solution,
molecules much be moving more slowly for the attraction between the solvent molecules (such as water) to
become more effective than the interaction between the solute and solvent molecules, therefore water will freeze
at a lower temperature.
6. Such ionic substances as sodium chloride (NaCl), calcium chloride (CaCI2), and the nonelectrolyte urea
(NH2CONH2) are frequently used to melt ice on streets and highways. Rank the three from high to low in
terms of their effectiveness per equal 1.0 m solutions in lowering the freezing point. Explain your ranking.
If the molalities are equal, the melting “power” is based on the number of ions/molecules each breaks into (in
other words, the “i” value). Calcium chloride breaks into 3 ions for every 1 formula unit (i-value = 3) and
therefore of the substances listed, it will be the most effective. Sodium chloride breaks into 2 ions for every 1
formula unit (i-value = 2), therefore NaCl will be less effective than CaCl2, but still more effective than urea,
which is a molecular compound, and just breaks into molecules (i-value of 1).
So ranking highest to lowest: Calcium chloride, sodium chloride, urea
7. Comparing pure water and a 1 m aqueous glucose solution, which has the higher value of each of the
following?
a. Vapor pressure
water
1 atm
b. Boiling point
glucose soln
c. Freezing point
water
d. Osmotic pressure
water
8. With respect to the same properties, how would a 1 m NaCl solution compare with the two solutions mentioned
in Problem 7?
a. Vapor pressure
lower vapor pressue
1 atm
b. Boiling point
higher boiling point
c. Freezing point
lower freezing point
d. Osmotic pressure
lower osmotic pressure
9 Describe benefits of using ethylene glycol with water in the cooling system of an automobile in the summer
months.
Cooling systems work by removing heat through evaporation (the heat is absorbed and causes evaporation of the
liquid…which removes the heat from the system as the gas condenses elsewhere). Creating a solution of water
and a non-volatile substance causes the vapor pressure of water to decrease, meaning the water needs to absorb
more heat to evaporate, therefore creating a more effective cooling system.
10 . Methanol, CH3OH, or ethylene glycol, CH2OHCH2OH, both nonelectrolytes, can each be used as antifreeze
in automobile radiators. If equal weights of each were added to the radiators of two cars, which one would
result in the greater freezing point depression?
Methanol
Equal weights would provide more moles of methanol (because it has a lower molar mass). Therefore the
molarity of methanol would be higher. Both of them have an i-value of 1 (non-electrolytes), so the substance with
a higher number of moles (molality) will have the greater freezing point depression.
11. Provide an explanation for each of the following.
a. NaCl is added to ice in an ice cream machine to freeze the ice cream.
NaCl causes the water/ice mixture to lower in temperature because the ice melts at a lower temp
(colligative property – freezing point depression – see #5). This allows the ice cream to get colder and freeze.
b. Sea water has a lower freezing point than fresh water.
Sea water is a solution, so the molecules need to be moving more slowly for freezing to take place. Again,
see explanation on #5.