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Colligative Properties – Unit 16
(See lecture handout or webpage for k values and standard phase change temperatures.)
1. Rock Salt is composed of the same ionic compound as table salt: sodium chloride. Answer the
following questions about this compound:
Formula
i value
Formula mass
2. A student puts 1.25 kg of ice in an ice cream maker with 350 g of rock salt. All the ice eventually
becomes liquid water as the rock salt dissolves. Solve for the molality of the solution, then for its
new freezing temperature.
GIVEN
WORK
SOLUTION CONSTANTS
Solvent:
Solute:
Tf =
i=
Kf =
MOLALITY:
FREEZING POINT:
3. A certain ice cream maker contains 1.70 kg of ice/water. Rock salt is added until the temperature of
the solution drops to -15.7ºC. Use this new freezing point to solve for the molality of the solution,
then for the number of grams of sodium chloride used.
GIVEN
WORK
SOLUTION CONSTANTS
Solvent:
Solute:
Tf =
i=
Kf =
MOLALITY:
GRAMS OF ROCK SALT:
4. An industrial ice cream maker contains 10.50 kg of ice/water and 70.0 moles of solute. The
freezing temperature of this solution is -37.1ºC. Use this new freezing point to solve for the i value
of the solute.
GIVEN
WORK
SOLUTION CONSTANTS
Solvent:
Tf =
Kf =
VAN’T HOFF FACTOR (i) OF SOLUTE.
Colligative Properties – Unit 16
LPChem after Johannesson
5. Indicate how many particles are formed when the following solutes dissolve. (What is “i”?)
# OF PARTICLES
SOLUTE
# OF PARTICLES
SOLUTE
sucrose (C12H22O11)
magnesium chloride (MgCl2)
sodium sulfate (Na2SO4)
methanol (CH3OH)
hydrochloric acid (HCl)
barium hydroxide (Ba(OH)2)
6. Find the boiling point of a solution containing 6.0 g benzene, C6H6, in 35 g of napthalene.
GIVEN
WORK
SOLUTION CONSTANTS
Solvent:
Solute:
Tb =
i=
Kb =
MOLALITY:
BOILING POINT:
7. Frodo’s feet are aching at the end of a long day trekking toward Mordor. Sam dissolves 50.0 g of
Epsom salt, MgSO4, in 1.5 kg of water so Frodo can soak his feet. What is the freezing point of
Sam’s solution?
GIVEN
WORK
SOLUTION CONSTANTS
Solvent:
Solute:
Tf =
i=
Kf =
MOLALITY:
FREEZING POINT:
8. A 2.00 gram sample of a very large biomolecule was dissolved in 15.0 g of carbon tetrachloride.
The boiling point of this solution was 77.85ºC. Calculate the molarity of the solution, then the molar
mass of the biomolecule.
GIVEN
WORK
SOLUTION CONSTANTS
Solvent:
Solute:
Tb =
i=
Kb =
MOLALITY:
MOLAR MASS:
PART B – APPLICATIONS
9. Salt is often used to remove ice from roads and sidewalks. Explain how this process works in terms
of colligative properties.
10. Which salt, NaCl or CaCl2, has a greater effect on freezing point? Explain.
Colligative Properties – Unit 16
LPChem after Johannesson