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Transcript
Chemistry Lab
Time Required: 50 min
Colligative Properties of Solutions: Freezing Point Depression
Objective: To calculate the freezing point depression of a salt/ ice mixture. Use the freezing point
depression to make ice cream.
Introduction:
A pure liquid substance freezes at a characteristic temperature, called the freezing, and its
temperature remains constant throughout the change of state. Suppose a solute is dissolved in a pure
substance. Is there a particular temperature at which the substance freezes? How is this
temperature related to the freezing temperature of the pure solvent? In this experiment you will
answer these questions by observing the cooling behavior of a solute.
The solvent in this experiment is water. Water is a liquid at room temperature, however, in
this experiment you will be using the crystalline solid state of water, ice. You will first record the
temperature of the frozen ice to note its temperature at the freezing point. Then, you will add a
measured amount of solute (salt) in the ice and determine the resultant temperature of the ice-salt
mixture.
The freezing point depression of the solution can also be calculated. However, this experiment
is only looking toward the qualitative aspects rather than the quantitative due to the nature of the
experiment and the desire to not use standard laboratory equipment.
EQUIPMENT AND MATERIALS:
NOTE: IT IS MANDATORY THAT YOU USE ONLY THE EQUIPMENT AND MATERIALS
PROVIDED ON YOUR BENCH. DO NOT USE ANY EQUIPMENT IN YOUR LAB POSITION,
INCLUDING YOUR APRON AND GOGGLES.
For a two Partner Group
Ziploc bags- 2 sandwich; 1
gallon
Measuring cup (station)
Ice (in cooler)
Milk*(at station)
Measuring spoons (station)
2-Styrofoam cups
2 plastic spoons
Vanilla (station)
Sugar (station)
Salt (station)
Thermometer
PROCEDURE:
1. Clean your workspace carefully.
2. Fill the gallon-sized Ziploc bag approximately ½ full with ice. Measure the temperature of the
ice and record in data table. ( you only need one of these per group)
3. Add 6 tablespoons of salt to the ice bag. Gently shake or knead the bag to make as uniform a
ice-salt mixture as possible. Measure the temperature of the ice-salt mixture.
4. Put 125 ml (1/2 cup) milk*, ¼ teaspoon vanilla, and 1 tablespoon sugar into sandwich-sized Ziploc
bag. Seal the bag and gently shake contents. Mass the mixture on the scale at my desk that
has been covered in plastic wrap. (You will need one bag of this for each lab partner)
5. Place the sealed, smaller bag into the large bag of ice-salt, distributing the ice evenly.
6. Record time (current) in your data table.
7. Each lab partner should hold two corners of the bag with the lab partners on opposite sides of
the table (if you get leaks- it will leak onto the table and not the floor) Shake vigorously.
After 5 minutes of shaking, check the appearance of the milk mixture. If the mixture is thick
and creamy in consistency (HINT: looks like ice cream!) Stop. If not of ice cream consistency,
continue shaking until desired consistency is achieved.
8. Record time when shaking procedure is finished.
9. Measure and record the final temperature of the ice-salt mixture.
10. Turn the ice cream out into a styrofoam calorimeter. Be sure that the ice cream does not
touch the outside of the bag (it will be really salty…yuck) obtain a clean spoon and consume the
ice-cream.
11. After eating, clean up your lab position thoroughly. Don’t leave it for Mama cause Mama’s
tired!
Data and Analysis:
Data:
Temperature of Plain Ice
Initial Temperature of Ice Salt Mixture
Mass of the milk-sugar solution
Initial Time
Final Time
_______________
_______________
_______________
_______________
_______________
Questions and Conclusions:
1. What change occurred in the temperature of the ice-salt mixture during the experiment?
2. What is freezing point depression? What is a colligative property?
3. How many moles of salt did you add to the ice (1TBS 20.0g)
4. The mass of ice used in this experiment was 1.00kg. Calculate the theoretical change in
temperature of the ice/salt mixture? How does your change in temperature compare?
5. Calculate the rate of freezing of the milk/sugar solution in grams/sec.
6. Give and examples of how we take advantage of the freezing point depression or boiling point
elevation of aqueous solutions in our everyday lives?
7. How does antifreeze added to a car’s radiator protect the car in both the winter and the
summer?
Use the constants for kb and kf and boiling/melting points to solve the following colligative property
problems.
8. Write the equation for the dissociation of calcium chloride. If 49.2 g of calcium chloride is
dissolved in 250.0g of ethanol, what will the new boiling point be? What will the new freezing
point be?
9. If 92 g of carbon tetrachloride is dissolved in 250.0g of ethanol. Calculate the new freezing
point and boiling point.
10. Which of the following compounds change the boiling point of water the most? Least? Explain.
NaCl
CS4
Al2(SO4)3