Colligative
Properties
NAME:
Hope Fluegel
TITLE
OF LESSON: Colligative Properties
LENGTH
OF LESSON: 50 minutes
NAME
OF COURSE: Chemistry
GRADE
LEVEL: High School
SOURCE
OF THIS LESSON:
Chemweb.
Colligative Properties of Solutions: Freezing Point Depression. Retrieved 3 Oct, 2006.
<http://chemweb.calpoly.edu/chem/125/125LabExp/FPDepression/
Doing>
TEKS
ADDRESSED:
Environmental Sciences:
(13) Science concepts. The student knows relationships among the concentration, electrical conductivity, and colligative properties of a solution. The student is expected to:
(B) interpret relationships among ionic and covalent compounds, electrical conductivity, and colligative properties of water; and
Chemistry
(12) Science concepts. The student knows the factors that influence the solubility of solutes in a solvent. The student is expected to:
(A) demonstrate and explain effects of temperature and the nature of solid solutes on the solubility of solids;
(B) develop general rules for solubility through investigations with aqueous solutions; and
PERFORMANCE
OR LEARNER OUTCOMES:
Students
will be able to:
¤
Define colligative
properties and give four examples of colligative properties
¤
Use experimental data to
estimate vanÕt HoffÕs factor
¤
Use the Freezing Point
Depression equation to calculate the Molarity of an unknown solutioin
RESOURCES,
MATERIAL AND SUPPLIES NEEDED:
- ice
- rock
salt
- various
solutions
- unknown
solution
- large
test tubes (17x100mm ones in the middle of the room)
- temperature
probe attached to Vernier interface
- 600-mL
beaker
- 50
mL graduated cylinder
SAFETY
CONSIDERATIONS:
Be careful with the
thermometers; they are delicate and can break. These solutions will become very
cold and should not be left in contact with studentsÕ skin for any amount of
time.
SUPPLEMENTARY MATERIALS,
HANDOUTS:
¤
Lab Guide
¤
Calculation Sheet
LESSON PLAN
|
Engagement: If you donÕt know the answer now, youÕll know by the end
of today! |
Why do they put ice on the roads in the winter? How does it work? |
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Exploration: Today weÕre talking about colligative properties. These are physical properties that depend only
on the amount of solute present in the solution, not on the properties of the
solute. Today, youÕll be exploring how solutes affect the freezing point.
YouÕll do the experiment on freezing points, and then weÕll talk more about
colligative properties. Students are given the lab procedure. We discuss
it as a group, then they complete it. |
|
|
|
Explanation: Discuss results. Show students what the
unknowns were. |
What did you find? What correlation do you see between the amount of solute
present and the freezing point? How did your value of the vanÕt Hoff factor compare to
the ideal one? |
Answers will vary. Allow
different groups to compare results. More solutes dissolved in
the solution lower the freezing point. (Give this a name: freezing point
depression.) |
|
Elaboration: There are three other colligative properties that have
to do with osmotic pressure, vapor pressure, and the boiling point. |
Do you think having more solutes in a solution will
cause boiling point depression or elevation? Why? Why do they put salt on the ground in the North? |
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Evaluation: Answer the following questions in your lab notebook: |
1. Explain, in your own words, why the actual van't Hoff factor is
typically lower than the ideal one. 2. Ethylene glycol (C2H6O) is added to automobile radiators to
prevent cooling water from freezing. What mass of ethylene glycol must be
added to 2.00 L of water to prevent the water in an automobile cooling system
from freezing at –20.0¡C? 3. When an aqueous solution is cooled to low
temperatures, part of the water freezes as pure ice. What happens to the
freezing point of the remaining solution when this occurs? Explain. |
|
Name_________________________
Colligative Properties
----- 
MATERIALS
Chemicals:
- ice
- rock salt
- various solutions
- unknown solution
-----
EQUIPMENT
- large test tubes (17x100mm ones in the middle of
the room)
- temperature probe attached to Vernier interface
- 600-mL beaker
- 50 mL graduated cylinder
-----
SAFETY PRECAUTIONS:
Goggles must be worn throughout the experiment.
-----
:BACKGROUND:
The purpose of this experiment is to demonstrate the effect of different solutes on the freezing point of water. In this experiment, we'll use the colligative property freezing point depression (ΔTfp) to determine the van't Hoff factor i for different known solutes and the molar mass of an unknown solute.
In the first part of the experiment, various solutes are added to water and the resultant freezing points of the solutions are determined. The actual value of the van't Hoff factor i, or number of dissolved particles per formula unit, is determined from the measured freezing point. This is then compared to the ideal van't Hoff factor. In the second part, the molar mass of an unknown solution is determined. Calculations involve use of the following equation for the change in temperature (in¡C) :
ΔTfp = Kf m ¥ i
Kf is a constant
for the solvent in ¡C/ m (weÕll name this constant later)
m is the molality of the solution in mol solute/kg solvent
i is the van't Hoff factor
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PROCEDURE:
As usual, all data should be
recorded in your lab notebooks. Each
pair at your table
will investigate one of four known solutions. You will share your data with
others at your table so that each pair has data on four solutions.
NOTE: The amount of solute per gram
of solvent will be indicated on the stock bottle of each solution, you will
record this information and use it to calculate the molality of the solution.
Preparation of Ice Bath
- Fill the beaker 3/4 full with ice.
- Cover the ice with 1/4 to 1/2 inches of rock salt.
- Stir this ice-salt mixture with a stirring rod and make
sure the temperature drops to -5¡C or below.
Determination of Freezing
Point of Pure Water
1. Make sure your temperature probe
is connected to the interface. Open LoggerPro using the FP.xmbl file found on
the desktop in the Chem 125 folder. Make sure you always use this file and not
"untitled".
2. Place a test tube that is 1/2
full of deionized water in the ice bath.
3. Place the temperature probe in
the middle of the water layer. Then press "collect" and stir the
water in the test tube gently with the temperature probe while LoggerPro
records the temperature.
IMPORTANT: Keep the
probe off of the bottom of the test tube. Why do you think you need to do this?
4. When the first ice crystals
appear on the inside wall of the test tube, you should observe the recorded
temperature to level off. This should be the freezing point of pure water.
Record the value in your lab notebook to the proper number of significant
figures.
Determination of Freezing
Points of Solutions:
1. Obtain about 25 mL of your
assigned solution in your graduated cylinder. Make sure you record the identity
of the solution as well as the given concentration.
2. Repeat steps 2-4 with this
solution to record the freezing point of the solution. You should do this at
least three times. Take the average of the measured freezing point data.
3. Share your freezing point data
for your solution with the others at your table*. Get freezing point data for
three other solutions from the other students at your table. Plot
representative freezing curves for each solution on the same graph, making sure
to clearly distinguish each curve. Turn in this graph with your report.
4. Calculate the molalities of each
solution from the given concentration information.
5. Using the measured freezing point
data and your calculated molalities, determine the value of i for each solution. Kf for water = 1.86¡C/m.
* You will want to share data after
transferring it to Excel. Probably the easiest way to do this is to first
transfer your data (copy & paste) from LoggerPro to Excel, give the Excel
workbook a distinct name that includes your computer number, then save your
Excel workbook and quit Excel. You can email the file to others.
Molar Mass Determination From
Freezing Point Data
1. Obtain an unknown solution, make
sure you record the unknown number as well as given concentration information
in your notebook. Unless instructed otherwise,
each partner should have their own unknown!
2. Freeze this solution in the same
manner as you did the previous solutions. Be sure to record the freezing point
temperature.
3. Calculate the molecular mass of
this solute based on the freezing point depression (i = 1.0).
WASTE DISPOSAL
Dispose of the solutions as
instructed.
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CALCULATIONS & REPORTING YOUR RESULTS UNLESS OTHERWISE INDICATED BY YOUR INSTRUCTOR, REPORT YOUR RESULTS IN THE FOLLOWING FORMAT: Determination of Freezing Points of Solutions:
Show your calculation of molality below:
Show your calculation for the theoretical ΔTfp for your solution: Create an overlay plot of Temperature versus Time containing both your data for pure water and a representative curve of your data for your solution. Also include on this graph as overlay plots representative data from each of the other solutions investigated at your cluster. Use the format that best illustrates these freezing curves. This graph must be submitted along with your report.
Determination of van't Hoff Factors of Solutions: You must share your results from above with everyone at your cluster to complete the table below. EACH SET OF PARTNERS must complete all the calculations for this table independently!!!!
For the solution that you investigated, show how you determined the experimental value of i below: For the solution that you investigated, explain how you determined the theoretical value i
Molar Mass Determination From Change in Freezing Point
Show how you determined the molar mass of
your unknown below: |
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