SPOKANEFALLS COMMUNITY COLLEGE
CHEMISTRY EXPERIMENT
FREEZING POINT DEPRESSION with LAB QUEST
NAME ______
INTRODUCTION
Some properties of solutions do not depend on the nature of the solute: they only depend on the number of solute particles (ions or molecules) relative to the number of solvent particles. We call these properties colligative properties. In these experiments we will focus on one particular
property, the freezing point depression.
Solutions of nonvolatile solutes have higher boiling points and lower freezing points than the solvents used to prepare the solutions. Both of these phenomena result from the fact that the vapor pressure of a solution is lower than the vapor pressure of the pure solvent. It is easy to see why.
The vapor pressure of a liquid is determined by the ability of particles at the liquid surface to escape into the vapor phase. When some of the spaces at the surface are occupied by nonvolatile solute particles, then fewer solvent particles can enter the vapor phase and the solution’s vapor pressure is lower than the pure solvent at all temperatures. In other words, the solution will required a high temperature before the vapor pressure reaches that of the atmosphere.
The same phenomenon (vapor-pressure lowering) that causes the boiling point elevation also leads to freezing point depression. The freezing point of a solution is lower than the freezing point of the pure solvent. Quantitatively, for a solute that is not an electrolyte, the lowering of the freezing point is described by the following equation:
DTf = Kf * m (for solutes that are non-electrolytes) (1)
where DTf represents the lowering of the freezing point (Tf), m is the molality of the solution, and Kf is the molal freezing point depression constant that depends only on the solvent. The degree of freezing point depression is proportional to the amount of solute present in the given mass of solvent. The difference between the freezing point of the solution and the solvent is called the “freezing point depression” shown by the label DTf. If DTf is calculated by subtraction the freezing temperature of the solvent from the solution, it will have a negative value. DTf = Tf,solution – Tf,solvent
The plots below represent typical cooling curves of a pure solvent and a solution.
The cooling curves below illustrate a very common phenomenon called: “supercooling”, which occurs when the temperature drops lower than the freezing point due to rapid heat loss in the cooling process. This can be minimized by careful stirring of the liquid to allow even cooling. As shown in these graphs, freezing points are still found in the similar way described above.
Cooling Curve for Pure Water Cooling Curve for a Solution
intersection of 2 slopes
freezing pt freezing pt
super super
cooling cooling
liquid liquid solid liquid liquid & solid
& solid solid
Time (sec) Time (sec)
The molal concentration of a solution represents how many moles of solute are dissolved per kilogram of solvent. For example, if 0.50 mol of fructose sugar is dissolved in 100 g of water, this would also be equivalent of 5.0 mol of fructose per kilogram of solvent and the solution would be 5.0 molal. The molality of the solution is then defined as:
(2)
The measurement of freezing point lowering is used to determine the molar mass of unknown solutes. If Equations (1) & (2) are combined, it can be derived that the molar mass of a solute is related to the freezing point lowering experienced by the solution and to the composition of the solution. Look at the following example:
The molal freezing point depression constant Kf for the solvent benzene is 5.12 oC-kg-mol-1. A solution of 1.08 g of an unknown in 10.02 g of benzene freezes 4.60 oC lower than pure benzene. Calculate the molar mass of the unknown. Equation (1) is used to obtain the molality of the solution:
m = DTf/Kf = 4.60 oC/5.12 oC kg mol-1 = 0.898 mol/kg
Equation (2), the number of moles of unknown present can be calculated:
m = 0.898 mol/kg = mol unknown/0.01002 kg solvent
moles of unknown = 0.00900 mol
Now, with the mol and mass of unknown, the molar mass is found by:
molar mass = 1.08 g/0.00900 mol = 120 g/mol
The previous example considered the effect of a nonionizing solute. If the solute instead ionizes, the effect on the freezing point will be larger. For example, a 0.1 m NaCl solution is effectively 0.1 m Na+ and 0.1 m Cl- ions.
Since we just discussed the workings of a nonionizing solute, what about the effects of an ionizing solute. When the solute is an electrolyte (ionizes) it is necessary to include the van’t Hoff, i, factor in the equation. Quantitatively, for an electrolyte solution the change in freezing point is described by the following equation:
DTf = Kf * m*i (for solutes that are electrolytes) (3)
where i is the van’t Hoff factor and represents the number of ions that would be produced after complete dissociation of the electrolyte in a very dilute solution (ideal conditions). We will use two different solutions of CaCl2 to determine the van’t Hoff factor using the known Kf value for water.
This experiment includes three parts. Part 1 is to determine the freezing point for acetic acid, an unknown, and calculations to determine the molar mass of the unknown. Part 2 is to determine the freezing point for water and two sucrose solutions. Part 3 is to calculate the van’t Hoff factor for CaCl2 from two different solution concentrations.
Data Analysis:
1. Obtain freezing points from the cooling curves and determine DTf for the pure solvents and the
various solutions. Draw the “arrows” on graphs to show how you found the “freezing point”.
2. Using the given Kf, determine the molality of the solute for the unknown additions in Part 1.
3. Calculate the average molar mass for the unknown in Part 1.
4. Calculate the van’t Hoff factor, i, in Part 3.
Lab Set
ADDIDATIONAL ProcedureS for LAB QUEST
1. Obtain a LAB QUEST and thumb drive. Connect the temperature probe to the Lab Quest.
Connect the power source to the LAB QUEST and connect to power outlet. Activate
the LAB QUEST by pushing the button on the upper left corner. A temperature reading should be
displayed on the “Meter Screen”. Insert the thumb drive in the USB slot located on the top end of
the LAB QUEST. Remove the stylus and use to check out the functions of this device.
2. Set up LAB QUEST for data acquisition. On home screen, right side, touch “Length”
a. reset RATE to “0.2” b. reset Length to “900” sec c. touch “OK”
You will be collecting data at 0.2 per second for up to 900 seconds. (this means it will collect data
every 10 secs for 15 min).
3. To collect data, lower right screen is a sideways pointing “triangle”. Touch triangle to start
collecting data, triangle will change to a “square”. Touch square to stop collecting data
4. Wipe clean the temperature probe with a wet paper towel and dry it before your next use.
** You may obtain graphs from the data files on your USB drive. To do this, you must use one of the
computers in the chem lab
Click the Logger Pro icon on the desktop to open the data files on USB drive. You must know how to enter appropriate instructions for this program to print temperature vs. time graphs using appropriate scales in each axis.
Or, you may left-click the data table shown on the Logger Pro screen and right-click to copy and then paste it into an Excell worksheet. You can than graph the data using the spreadsheet program.
It is best to print out each graph separately or no more than 2 graphs per page. An example of acceptable computer generated graphs are shown below.
prepared using Logger Pro prepared using Excell Spreadsheet
PROCEDURE
PART 1 Freezing point depression of acetic acid
In this experiment, you will obtain the cooling curve of a pure solvent, acetic acid and the curves after an unknown solute has been added to the solvent. From the measurements of DTf, you will be able to determine the molecular mass of the unknown.
A. Freezing point determination of pure acetic acid (HAc)
1. Fill a 600-mL beaker 1/2 full of tap water. Water is close to 35 oC. Warm water in another
beaker placed on a hot plate. This will be used to “thaw” your test tube after it freezes.
Fill another large beaker with ice cubes
2. Using a 50-mL graduated cylinder, carefully measure and add 30-mL of HAc to a large test tube
3. Place the test tube into the beaker making sure at least ¾ of the level of the liquid in the tube is
below the level of the water in beaker. Let the test tube equalize to the water temperature for
a brief moment of time
4. Insert the temperature probe into the test tube and start collecting data
5. While the data is being collected swirl the test tube. Careful not to lose any contents from the
test tube nor add any contaminants to the test tube
6. Begin by adding 3-4 ice cubes to the beaker. When cubes have almost melted, add more
cubes. When those cubes have almost melted, add more cubes. You need to control this
cooling process in slow manner. Keep adding ice cubes as needed but not too quickly
7. You will start to see solid, crystals, begin to form. Keep swirling the test tube. After the solid has
formed, record data for approximately 2 minutes
8. Remove the temperature probe, empty and refill your 600-mL beaker
9. Melt the HAc in a warm water bath. DO NOT dispose of the contents in your large test tube. As
this HAc solvent will be used in the next 2 parts of PART B
10. Create a file name and save data on flash drive
B. Freezing point determination of a HAc solution and an unknown
1. Refer to #1 from Part A
2. Weigh 1.000 g of unknown. This will then be added to the test tube when the contents have
completely melted. Be sure the unknown has totally dissolved, stir well
3. Place the test tube into the beaker making sure at least ¾ of the level of the liquid in the tube is
below the level of the water in beaker. Let the test tube equalize to the water temperature for
a brief moment of time
4. Insert the temperature probe
5. Repeats steps 4-8, from A above
6. Melt the HAc in a warm water bath. DO NOT dispose of the contents in your large test tube. As
this HAc solvent will be used in the next part
7. Create a file name and save data on flash drive
8. Refer to #1 from Part A
------
9. Weigh 2.000 g of the same unknown. This will then be added to the test tube when the contents
have completely melted. Be sure the unknown has totally dissolved, stir well
10. Insert the temperature probe
11. Again, repeat steps 4-8, from A above
12. Melt the HAc in a warm water bath. NOW, properly dispose contents of your test tube
13. Create a file name and save data on flash drive
14. Determine the freezing point using the computer graph
15. Calculate the average molar mass of the unknown from your data
Analysis of the Data
You need to determine the temperature at which the pure solvent froze. Then you must determine the temperature at which the first and second mixtures froze. This is used to find the freezing point depression in part B. The density for HAc is 1.049 g/mL and the Kf (HAc) = 3.90 oC-Kg-mol-1. The density of the unknown is 1.075 g/mL. The data is used to determine the freezing point depression of the unknown solvent. Using the Kf you determined in part 1, you can now determine the molar mass of the unknown. Molar mass of the unknown is in the range of 110 – 135 grams.
**Outside of this range will result in the loss of 5 points. The point deduction is as follows:
±2.0 % No Deduction ±4.0 % 1 Point ±6. % 2 Points
±8.0 % 3 points 8.0% 5 points
Preparation of rock salt-ice water bath as used for Part 2 & Part 3
Fill a 600-mL beaker 2/3 with layers of ice and rock salt. Add 20-35 mL of tap water to the beaker.
Stir this mixture thoroughly with a stirring rod. Check the bath temperature with a regular thermometer. Maintain a constant ice-bath temperature of -10 oC or lower throughout the data collection. Occasionally
remove excess water as needed.
PART 2 Freezing point depression of water and nonelectrolyte sucrose solutions
In this part, you will determine the freezing point of two sucrose solutions of known molalities: 0.5 and 1.0 m. Because the freezing point of water is 0oC, you will need to prepare a rock-salt ice bath so temperatures below 0oC can be attained. After preparing the bath, you are now ready to start. You will first determine the freezing point of pure water and then the 2 sucrose solutions.
C. Freezing point and Kf determination of pure water
1. Add 20-mL of water to a larger test tube
2. Insert the temperature probe
3. Place the tube in the rock-salt ice bath and start collecting data
4. While the data is being collected, constantly swirl the test tube in the ice bath
5. You will start to see solid, crystals, begin to form. Keep swirling the test tube. After the solid has
formed, record data for approximately 2 minutes
6. Remove the temperature probe
7. Melt the sample and properly dispose of it
8. Create a file name and save data on flash drive
9. Determine the freezing point of WATER using the computer graph.
10.Calculate the Kf for water from the data collected. Knowing that the Kf,water = 1.86 oC-Kg-mol-1,
calculate the relative % error between your experimental value and the know value.