Colligative Properties please follow up the guide lines for doing my report plz. Also, I upload the data for you. LAB EXERCISE 2 Colligative Properties: Fi
Colligative Properties please follow up the guide lines for doing my report plz. Also, I upload the data for you. LAB EXERCISE
2
Colligative Properties: Finding the van’t Hoff Factor
OBJECTIVES
• Learn to use colligative properties to predict the freezing point of a solution
• Determine the van’t Hoff factor for an ionic salt using a freezing point depression
measurement
• Find the molar mass of an unknown compound using a freezing point depression
measurement
EQUIPMENT AND MATERIALS
• Calcium chloride dihydrate
• Ammonium chloride
• Plastic test tubes
• Test tube holder
• Glass stirring rod
•
•
•
•
Thermistor
250 mL beaker
Rock salt
Ice
INTRODUCTION
The properties of a solution differ from those of a pure solvent due to interactions that take place
between the solute particles and solvent molecules. These properties are called colligative
properties. Colligative properties are dependent upon the number of particles (ions or
molecules) dissolved in the solvent and not on the identity of the particles. This experiment will
examine various phenomena involved with freezing point depression.
PART I – SAMPLE PREPARATION
1. Make a data table in your lab notebook to record: Mass of test tube, Mass of test tube +
water, Mass of test tube + water + CaCl2, Mass of water, Mass of CaCl2, freezing point
(Tf), freezing point depression (∆Tf).
2. Use the following table to make 3 calcium chloride solutions in test tubes, as well as a
water blank. The amounts listed in the table are approximately what you should measure
out, but record the actual amounts in your data table. For the water, add ~5 mL to a
tared graduated cylinder and record the actual mass of water in your table.
1
Solution #
Mass of Solvent (g)
Mass of Solute (g)
0
~5
0
1
~5
~0.4
2
~5
~0.2
3
~5
~0.05
3. Using the actual amounts of solute and solvent in your solutions, calculate and record in
your lab notebook the predicted freezing point of each solution. Be sure to show your
work for one of the calculations in your sample calculations section in your lab report.
PART II – DETERMINATION OF FREEZING POINTS
1. Prepare an ice bath by filling your 250 mL beaker with ice. Fill about half way with tap
water. Cover the top of the ice with about 1/4 inch of rock salt and stir with your glass
rod. If it ever looks like salt has settled on the bottom of the beaker, give it a stir to keep
the salt suspended in solution.
2. Set up a Microlab experiment to read temperature (with your thermistor) and time. Put
temperature on your y-axis and time on your x-axis in the graphing area.
3. Suspend the thermistor in your test tube containing only water, making sure that the tip
does not touch the glass. Lower the test tube into the ice bath. Agitate the contents of
the test tube using the thermistor continuously.
4. Continue to agitate the contents of the test tube until freezing occurs. Record the
freezing point (Tf) of the solution. Note – you may experience supercooling in your
sample, where the temperature dips below the freezing point then come up rapidly. If this
occurs, record the highest temperature after the rapid rise.
5. Warm the sample in warm water to melt the ice back into solution then dump your
solution into a waste beaker.
6. Repeat steps 3-5 for your other solutions, being sure to record each of the freezing
points for each trial. Also be sure to stir the ice bath regularly.
7. Repeat the steps in Part I and Part II using ammonium chloride as your salt. You can
skip the freezing point measurement for pure water, solution 0, and use the result from
the first trials.
PART III – CALCULATING THE VAN’T HOFF FACTOR
1. Calculate the ∆Tf for each sample by subtracting the freezing point of each solution from
the freezing point of your water sample.
∆Tf = Tf(H2O)-Tf(solution)
2
2. Calculate the molality of each of the salt solutions using your recorded values for mass
of water and mass of salt.
3. Calculate a value of the van’t Hoff factor, i, for each solution concentration.
4. In Excel, prepare a separate graph for each salt to calculate the value of i. Your graph
will be determined by the following relation:
y = mx + b
ΔTf = iKf m
5. ∆Tf values will be plotted on the y-axis, and the product Kf · m will be plotted as the x-axis
value. (Use 1.86 °C/m for Kf) Calculate your slope to find the value of i for each salt.
DATA ANALYSIS
PART II
1. Discuss any differences between your predicted values for the freezing point of the
various solution concentrations and your measured values. Show one example of all
calculations including an average %error for your solutions.
2. Discuss any trends seen in your %error between what you calculated for theoretical ∆Tf
and your measured ∆Tf.
PART III
1. Discuss any trends seen in the values of i as calculated in step 3.
2. Compare the slope found in step 5 and compare it to the values calculated in step 3.
3. Discuss any differences in the values you found for i experimentally and the value you
used when calculating a theoretical freezing point. What does your experimental value
for i suggest about what is occurring in solution?
4. Are there any differences between the the two salts? Does one salt have less error than
the other from the expected value of i? What is the reason for that?
QUESTIONS
1. When doing freezing point depression calculations we always express the concentration of
the solution in molality. Why do we not use molarity?
2. What is supercooling, and why does it occur? Is it possible to “superheat” a solution?
3. What is ion-pairing and how might it affect the results you saw in your experiment?
CONCLUSIONS
Reiterate your procedures briefly (including any changes you made). Summarize any results
that you may have calculated (with errors if applicable). You don’t need to include the raw data,
but if you calculated an average over several trials, state the average (not each trial). Discuss
the significance of your results for each part. If your experimental error is small or large
comment on what this means. Speculate on possible sources of error.
3
experiments with CaCl2 Solutions
mass of testmass
freezing
mass of Test-tube Test-tube
mass of
Freezing
initial
Time
tube water
of
point
beaker
mass
and water
water
Point
temperature elapsed
and CaCl2
CaCl2
depression
0 161.064
17.979g
21.05
N/A
3.071
0
1.01
0
22.85
292.92
1 161.064
17.979g
22.304
22.387
4.325
0.083
-3.42
4.43
21.625
880.38
2 161.064
17.979g
21.751
21.775
3.772
0.024
-2.77
3.78
22.174
193.92
3 161.064
17.979g
21.935
21.974
3.956
0.039
0.59
0.42
20.205
167.91
Lab Report notes, more information on
D2L
introduction
purpose, introduce colligative
properties freezing point depression.
real world example
procedure and observations
3rd person past tense
data
figures must have captions
experiments with NH4Cl Solutions
mass of test- mass
freezing
mass of Test-tube Test-tube
mass of Freezing
initial
Time
tube water and
of
point
beaker
mass
and water
NH4Cl
Point
temperature elapsed
NH4Cl
water
depression
0 161.064
17.979g
21.05
NA
3.071
0
1.01
0
22.85
292.92
1 161.064
17.979
21.947
22.067
3.968
0.12
-0.85
1.86
18.271
340.375
2 161.064
17.979
20.498
20.755
2.519
0.257
-3.09
4.1
18.044
144.86
3 161.064
17.979
20.801
20.852
2.822
0.051
0.26
0.75
19.111
116.37
1
data analysis
examples of calculations
show everything
conclusion
error analysis
what could have been done better
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Untitled1 : Thermistor A / Data Source 3 vs. Time
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