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COLLIGATIVE PROPERTIES - FREEZING POINT DEPRESSION
INTRODUCTION
As would be expected, when a solute is dissolved in a solvent, the resulting properties of the solution are
very different than those of the pure solvent. Solutions exhibit a lower vapor pressure, a higher boiling point,
and a lower freezing (melting) point than the pure solvent and also exhibit osmotic pressure. These properties
are called colligative properties and their magnitude depends entirely on the concentration of moles of solute
particles and not on the chemical identity of the solute particles. When dissolved in a given solvent and in a
given quantity of the solvent, a mole of sodium ions, a mole of sulfate ions, a mole of sugar molecules and a
mole of albumin (a protein) molecules, would each lower the vapor pressure, raise the boiling point, or lower
the freezing (melting) point to the same magnitude and each solution would also have the same osmotic
pressure. This phenomenon is closer to ideal in dilute solutions rather than concentrated ones. Colligative
properties are useful in counting moles of a solute, even when the identity of the solute is not known. If one can
count the number of moles of a substance in a given number of grams of that substance, the molar mass is easily
calculable.
The magnitude of the lowering of the freezing point is directly related to the concentration of the
solution and can be expressed mathematically by the expression, ΔTf = -kf m, where ΔTf represents the change
in the freezing point (Tf of the solution minus the Tf of the pure solvent). Since the freezing point of the solution
is always lower than that of the pure solvent, Tf is always negative and thus the minus sign. Kf is a constant for
any given solvent and is called the molal freezing point depression constant. The value for kf for water is 1.86
o -1
C or oC kg solvent/mol of solute. The m represents the molality of the dissolved solute particles in the
solution and is the moles of solute particles per kilogram of solvent. Molality and mole fraction are used for
vapor pressure, boiling point and freezing point studies because they are independent of temperature as opposed
to molarity, which is temperature dependent. The greater the concentration of the solution, the lower the
freezing point will be.
In solutions made with solutes that dissociate when dissolved in water, the molality of dissolved
particles is always greater than the molality of the solution. In other words, NaCl forms Na+ and Cl- in water so
a 1 molal NaCl solution would behave as a 2.0 molal solution. Sugar molecules dissolve without dissociation,
so a 1 molal sugar solution behaves like a 1 molal solution. In fact this methodology is useful for determining
the degree of dissociation for weak electrolytes.
Anything dissolved in water serves as an antifreeze and the more antifreeze the lower the freezing point.
When salt is put onto sidewalk ice, it dissolves to form a very concentrated solution. If the surrounding ice has
a temperature higher than the freezing point of the salt water, it will melt. Melting, of course, requires energy
and the energy comes from the surroundings making them colder.
In this exercise, the freezing points of a variety of aqueous solutions will be measured and the molar
mass of glycerin will be determined using freezing point depression data.
SAFETY CONSIDERATIONS
None of the materials used in this exercise are hazardous.
DISPOSAL OF MATERIALS
All materials can be flushed down the drain with water.
f. p. depression
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Name____________________date_______
PROCEDURE
I. Preparing an ice bath
1. Place about 300 mL of crushed ice in a 600 beaker.
2. Add about 6 grams (heaping teaspoon) of NaCl to the ice and stir the mixture vigorously and record
the temperature to the nearest 0.1 degree.
3. Add three more successive 6 g samples of NaCl to the mixture, stirring and recording the temperature
after each addition.
II. Freezing point of pure water (calibration of the thermometer)
1. Place about 10 mL of distilled water into a clean large test tube, equipped with a thermometer and
copper wire stirrer which fits around the thermometer..
2. Chill the water in the test tube with the ice bath until there is some ice in the tube. Stir the water in the
tube vigorously. DO NOT FREEZE IT SOLID. Record the temperature of a liquid - solid
equilibrium as the freezing point of the pure water.
III. Freezing points versus concentration of NaCl
1. To 10.0 mL (0.010 kg) of distilled water in a clean test tube, add 0.59 grams(0.01mol) of NaCl.
2. Measure and record the freezing point of the solution by chilling with vigorous stirring in the ice bath.
The more water that freezes in the mixture, the more concentrated the solution becomes and the lower
the freezing point becomes. Therefore, the desired freezing point is determined when the first crystal
of ice forms and the concentration of the solution is known. This is very difficult to determine. It is
best done by chilling the solution with vigorous stirring until there are some ice crystals in the tube and
then removing the tube from the ice bath, allowing the solution to warm while stirring vigorously and
reading the temperature when the last crystal of ice melts. The tube can be lowered into the ice bath
and the process repeated. Several reading can thus be averaged. Some practice is usually required to
obtain good freezing point data.
3. Add another 0.59 g sample of NaCl to the 1.0 m NaCl solution and repeat the above freezing point
measurement. Record the data.
4. Repeat step 3 making the solution 3 molal.
IV. Freezing point of a sugar solution.
1. Determine and record the freezing point of a 2.0 molal (342 g/mol) sugar solution using the above
methodology.
To 10.0 mL (0.010 kg) of distilled water in a clean test tube, add 6.84 grams(0.02mol) of sugar.
V. Molar mass of glycerin.
1. Accurately weigh out a 2.50 to 3.0 gram sample of glycerin into a clean large test tube. BE
CAREFUL NOT TO SPILL GLYCERIN ON THE BALANCE.
2. Add 10.0 mL of distilled water to the glycerin and dissolve it.
3. Determine and record the freezing point using the above methodology.
DATA, CALCULATIONS AND ANALYSIS
I. Preparing the ice bath
Temperature with 6 g NaCl. _____ 12 g _____ 18 g _____ 24 g _____
Why is there a minimum temperature that can be achieved with ice in salt water?
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What is the minimum temperature in degrees Celsius? ________ in degrees Fahrenheit? _________
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f.p.depression
Name____________________date_______
II. Freezing point of pure water (calibration of individual thermometers)
What is the freezing point of pure water? ________
What temperature does the thermometer read at this temperature (nearest 0.1 degree)? ________
III. Freezing point versus concentration of NaCl
0.5 m
Reading # 1 ______ # 2 ______
Average _______
ΔTf ______
1.0 m
Reading # 1 ______ # 2 ______
Average _______
ΔTf ______
2.0 m
Reading # 1 ______ # 2 ______
Average _______
ΔTf ______
What does this data indicate regarding the dissociation of NaCl in water?
IV. Freezing point of a sugar solution
1.0 m
Reading # 1 ______ # 2 ______
Average _______
ΔTf ______
How does this data compare to the ΔTf of the 1.0 m NaCl solution
What does this data indicate regarding the dissociation of sugar in water?
V. Molar mass of glycerin
Mass of glycerin __________
Freezing point of solution. Reading # 1 ______
# 2 ______
Average _______
ΔTf ______
Molality of glycerin solution using Tf= Kf m _________
Molar mass of glycerin using your experimental data __________
The formula for glycerin is C3H8O3. What is the % error in your determination? ________
QUESTIONS
1. Why does ice water get colder when salt is added and how does salt help melt sidewalk ice if it
lowers the temperature?
2. To what minimum temperature would a radiator be protected if equal volumes of ethylene glycol
(permanent antifreeze) (density 1.11 g/mL) and water were mixed. Assume that ethylene glycol does
not dissociate and has a formula, C2H6O2.
3. A 7.85 g sample of a compound with an empirical formula C1H1 is dissolved in 172 g of benzene.
The freezing point of the solution is 1.50 C below that of pure benzene. Assuming that the
compound does not dissociate, what is the molar mass and molecular formula of this compound?
The freezing point depression constant for benzene is 5.12 C/m.
_____________________g/mole
C_____H_____