CHM 112 American College of California Molar Mass of Solid Lab Report

Using Freezing Point Depression to Determinethe Molar Mass of a Solid
OBJECTIVES
• To determine the freezing temperature of a pure liquid solvent, cyclohexane, (C6H12).
• To observe and measure the effect of a solute on the freezing point of the solvent.
• To calculate the molar mass of a nonvolatile, nonelectrolyte solute from the freezing point depression.
INTRODUCTION
A pure liquid solvent such as water or ethanol has many characteristic physical properties, e.g., freezing point, melting
point, density, vapor pressure, specific heat capacity, etc. When a solute is dissolved in a solvent, however, the solvent
assumes physical properties that are no longer definite, but depend on the amount of solute added. The vapor pressure of
the solvent in the solution decreases, causing the freezing point of the solvent to decrease, the boiling point of the solvent
to increase, and osmotic pressure of the solvent to increase. These four physical properties are colligative properties;
they depend on the ratio of solute and solvent particles, not on the chemical identity of the solute. The changes in the
properties of a pure liquid solvent that result from the presence of a nonvolatile solute are portrayed by the phase diagram
in Figure 1.
Figure 1. Vapor pressure lowering, freezing point depression, and boiling point elevation of solution vs. pure solvent.
It is relatively straightforward to investigate the quantitative effect of a solute on the freezing point of a solvent. The
equation that defines this relationship is:
DTf = Kf ´ m
(Eqn. 1)
where DTf is the freezing point depression, Kf is the freezing point depression constant for a particular solvent, and m
is the molality of the solution (in mol solute/kg solvent).
As can be seen from Eqn. 1, the magnitude of the freezing point depression is directly proportional to the molality, m, of
the solute in solution. The proportionality is a constant, Kf , characteristic of the actual solvent. A collection of molal
freezing point and boiling point constants for several solvents is shown in Table 1.
FREEZING PT DEPRESSION/MOLAR MASS OF A SOLID | Aug18
Table 1. Molal Freezing Point and Boiling Point Constants for Several Solvents
Solvent
Melting Point (˚C)
Kf (˚C/m)
Boiling Point (˚C)
Kb (˚C/m)
Acetic Acid
16.6
3.90
117.9
3.07
Benzene
5.5
4.90
80.1
2.53
Carbon Tetrachloride
-23
30.0
76.5
5.03
Cyclohexane
???
20.0
80.7
2.69
-117.3
1.99
78.5
1.22
0.0
1.86
100.0
0.512
Ethanol
Water
Temperature (C)
In this experiment, the freezing points of a selected pure solvent (cyclohexane, C6H12) and a solute-solvent mixture are
measured. The freezing points of the solvent and solution are obtained experimentally from a cooling curve – a plot of
temperature vs. time. An ideal plot of the data appears in Figure 2.
Solvent
ΔTf
Solution
Time (seconds)
Figure 2. Cooling curves for a solvent and solution
Ideally, the cooling curve for a pure solvent reaches a plateau at its freezing point, so extrapolation of the plateau to the
temperature axis (y-axis) determines the freezing point. Unfortunately, the cooling curve for a solution does not reach a
plateau, since as some of the solvent freezes, the remaining solution becomes increasingly concentrated. Consequently,
the solution freezing point must be determined by the intersection of two straight lines drawn through the data points
directly above and below the freezing point (see Figure 2).
In Part A of this experiment, students will determine the freezing temperature of pure cyclohexane graphically.
In Part B, students will determine the freezing point depression of a solution containing a known mass of a nonvolatile
solute, biphenyl, in cyclohexane. Using the experimentally obtained freezing point depression, the masses of both solvent
and solute, and the known molal freezing point constant for cyclohexane, students will rearrange Eqn. 1 in order to
calculate the molar mass of biphenyl.
FREEZING PT DEPRESSION/MOLAR MASS OF A SOLID | Aug18
Experiment Procedure
MATERIALS: LabQuest with Temperature Probe, 25-mL graduated cylinder (short), two-hole rubber stopper, metal
stirring wire, two (2) 600-mL beakers for ice baths, 400-mL beaker for warm water bath, cyclohexane,
ice, NaCl (for ice bath), sample bottle of biphenyl (solute), spatula, personally-owned flash drive
1. Check out LabQuest PDA from instructor. Connect
the Temperature Probe to LabQuest Channel 1 (this
may already be done). Choose LabQuest App from
main menu, if necessary. The Meter screen should
be displayed – if it is not displayed, tap “Cancel”,
then tap the Meter icon in the upper left corner.
drop-down menu, select “Clear All Data”. Tap the
Meter icon to return to the Meter screen. From the
File menu, select “New”.
3. On the Meter screen, tap Rate. Change the Duration
to 10 minutes and the data-collection Rate to 4
samples/minute, and tap “OK”.
2. If necessary, discard previous student data: Tap the
Data icon in the upper right corner. From the Table
Part A. Determine the Freezing Temperature of Pure Cyclohexane
4. Obtain a short 25-mL graduated cylinder and a twohole rubber stopper, making certain that the stopper
will fit snugly in the mouth of the cylinder.
5. Rinse the graduated cylinder with 3 – 5 mL of
cyclohexane; dispose of the cyclohexane in “nonhalogenated Organic Waste”. Measure 20.0 mL of
cyclohexane in the graduated cylinder and secure the
cylinder in one of the 600-mL beakers (which has
been placed on a layer of paper towels). Calculate
the mass of cyclohexane from its published density.
Record the mass on the Report Sheet.
6. Insert metal stirring wire in the outside hole of the
stopper; insert the temperature probe in center hole.
7. Insert the stopper-stirrer-probe assembly in the
mouth of the graduated cylinder, making sure that
the temperature probe is immersed in the liquid, and
that the stirring wire can move freely in the cylinder.
8. Prepare an ice-salt-water bath in the second 600-mL
beaker by mixing approx. 40 cm3 of NaCl in 150 mL
water, adding ice to create at least 500 mL of
mixture.
9. Simultaneously pour the ice-salt-water mixture into
the beaker containing the 25-mL cylinder and tap the
green arrow (lower left screen) to start data
collection for Run 1. Make sure the ice bath level
outside the cylinder is higher than the cyclohexane
level inside the cylinder, as demonstrated by your
instructor.
10. Continuously stir the cyclohexane with a slow upand-down motion of the stirring wire until the liquid
begins to freeze and the stirring wire cannot move.
Do not splash the cyclohexane up the walls of the
cylinder. Continue data collection for the ten-minute
duration of the experiment – the temperature should
fall below 0 ºC.
11. When data collection is complete, remove the
graduated cylinder from the ice bath, and use a warm
water bath (in 400-mL beaker) to melt the
cyclohexane. Carefully remove the stopper
containing the temperature probe and stirring wire,
and place on a clean paper towel. Do not dispose of
the ice-salt-water bath, as it is needed for Part B.
12. The freezing temperature of the pure solvent is
initially determined by finding the mean temperature
in the flat portion of the graph with nearly constant
temperature. Note: If no portion of the graph
appears relatively flat (relatively constant
temperature over 1 – 2 minutes), consult your
instructor.
a. Select the data point at the beginning of the flat
portion of the graph and drag across the flat
portion to select the region.
b. Choose “Statistics” from the Analyze menu.
c. Record on the Report Sheet the mean (average)
temperature as the freezing temperature of pure
cyclohexane.
13. Store the temperature vs. time data from Run 1 by
tapping the File Cabinet icon. Note that Run 2 will
immediately become available. (The data for both
runs will be exported to a flash drive at the
completion of the experiment.) Obtain instructor’s
approval of the data before continuing on to Part B.
Part B. Determine the Freezing Point Depression of a Solution containing Nonvolatile Solute
14. Obtain a sample bottle of crystalline biphenyl solute.
Tare a small weigh boat; place 0.15 – 0.2 g of solid
solute in the weigh boat and record its mass to the
nearest 0.0001 g. Carefully transfer all the biphenyl
into the thawed solvent in the graduated cylinder,
making sure not to leave crystalline solute on the
FREEZING PT DEPRESSION/MOLAR MASS OF A SOLID | Aug18
walls of the cylinder. Re-insert the stopper-stirrerprobe assembly and use the stirring wire to gently
agitate the solution until all solute dissolves. Again
secure the cylinder in an empty 600-mL beaker.
15.
Replenish the ice-salt-water bath in the second 600mL beaker (if necessary).
16.
Repeat Steps 9 – 10 for Run 2. When data
collection is complete, examine the data pairs on the
displayed graph. The freezing point of the solution
may be approximated by finding the temperature at
which the mixture appears to start to freeze. Unlike
pure cyclohexane, the mixture results in a gradual
decrease in temperature during freezing. As you
move the examine line, the temperature and time
values are displayed to the right of the graph. Locate
the initial freezing temperature of the solution, as
shown in Figure 3, and record this approximate
freezing temperature of the solution in your data
table.
17.
Tap Run 2 and select All Runs to view both
Runs on the same graph.
18.
To save the data from Runs 1 & 2 as a text file to a
flash drive, insert the flash drive directly into the
USB port on the LabQuest unit, then select “Export”
from the File menu. Tap on the USB flash drive
icon, then tap on the name field and enter the file
name. Tap “Done” to return to the Export screen,
then tap “OK” to export the file as a text (.txt) file.
(Alternatively, export the file directly to the lab
computer using the USB computer cable provided
by your instructor.) Your instructor may demonstrate
the Export function using a test data file.
19. Obtain instructor’s approval of the LabQuest data
before disposing of the solution. Check flash drive
to confirm that file is present and not corrupted
PRIOR to completing Step 20.
20.
To shut down the LabQuest, select “Quit” from the
File menu, then select “Shut Down” from the
System folder.
21.
Disposal and Cleanup: Dispose of cyclohexane
solution in “non-halogenated Organic Waste”.
Clean graduated cylinder with soap and water, rinse
with distilled water, and return to original storage.
Dispose of water baths in sink, rinse glassware, and
return to original storage. Wipe down temperature
probe and return LabQuest PDA and Probe to
instructor.
Freezing Point
Time
Figure 3. Approximation of Solution Freezing Point.
Data Analysis in Excel
1. To open the exported .txt file in a spreadsheet
program such as Excel, confirm the program’s file
browser is set to look for all file types, and select
your text file. (In Excel, the Text Import Wizard will
guide the user through a series of questions.
Continue to tap “Next” until the import process is
complete.) Initially, the data for Run 1 and Run 2
will be listed separately. Using Excel, manipulate
the data to plot temperature vs. time data to show
both Run 1 and Run 2 on the same “cooling curve”
graph. Label the axes, title the graph, and print the
raw data and the graph on separate pages.
2. Experimentally, the freezing points of the pure
solvent (Run 1) and solution (Run 2) should be
obtained from the cooling curves that were printed.
Refer back to the idealized plot of the data from
Runs 1 & 2 in Figure 2; the cooling curve for pure
cyclohexane achieves a horizontal plateau at its
freezing point, so that extrapolation of the plateau to
the y-axis determines the freezing point. The
average freezing point of cyclohexane can then be
determined from both analysis methods (LabQuest
and printed graph).
3. As noted in the Introduction and Step 16 above, the
cooling curve for the solution does not achieve a
horizontal plateau, but gives a shallow negative
slope as the solvent freezes out of solution
(increasingly concentrating the remaining solution).
From the cooling curve, the freezing point of the
solution (Run 2) should be determined at the
intersection of two straight lines drawn through the
data points above and below the freezing point
(review Figure 2). The average freezing point can
then be determined from both analysis methods. The
freezing point depression is determined by
subtraction.
FREEZING PT DEPRESSION/MOLAR MASS OF A SOLID | Aug18
Using Freezing Point Depression to Determine Molar Mass of a Solid
Report Sheet
Date: __________
Name: ______________________________ Partner(s) Name:__________________________
PART A. Freezing Point of Pure Cyclohexane
Volume of cyclohexane (mL): ______________
Density of cyclohexane (g/mL): ______________
Mass of cyclohexane (g):
kf for cyclohexane (˚C/m):
______________
From LabQuest
Data Analysis
From Extrapolation
of Printed Graph
20.0
Instructor’s
Approval of
LabQuest Data
Freezing Point of
Cyclohexane (˚C)
Average Freezing Point
of Cyclohexane (˚C)
PART B. Freezing Point Depression of Cyclohexane Solution containing Unknown Solute
Mass of cyclohexane (from Part A): ________________
Mass of added biphenyl (g):
________________
From LabQuest
Data Analysis
From Analysis of
Printed Graph
Instructor’s
Approval of
LabQuest Data
Freezing Point of
Solution (˚C)
Average Freezing Point
of Solution (˚C)
(Attach printed
graph and raw data)
Calculations
Freezing point depression, ΔTf (˚C ):
_________________
Mass of cyclohexane in solution (kg):
_________________
Moles of biphenyl (mol):
_________________
Experimental molar mass of biphenyl (g/mol):
_________________
Show detailed calculation:
FREEZING PT DEPRESSION/MOLAR MASS OF A SOLID | Aug18
Using Freezing Point Depression to Determine Molar Mass of a Solid
Post-lab Questions
Date: __________
Name: ______________________________ Partner(s) Name:__________________________
1. Molar mass of biphenyl determined from Part B:
__________________
True molar mass of biphenyl:
__________________
Percent Error in molar mass:
__________________
Show detailed calculation:
2. If some of the pure cyclohexane solvent vaporized during data collection during experimental procedure Part A, how
would the freezing point of pure cyclohexane be affected (too high, too low, unaffected)? Justify your answer.
3. If the temperature probe was mis-calibrated by +0.10 ˚C during both experimental procedure Part A and Part B,
would the reported moles of biphenyl in the solution be too high, too low, or unaffected? Justify your answer.
4. Consider the following errors that can occur during Part B of the experiment. How would each of these errors affect
the experimental molar mass of a solute (greater than true molar mass, less than true molar mass, unaffected)?
Justify your answers.
a. Some of the solid solute adhered to the side of the test tube during data collection.
b. Some of the cyclohexane solvent vaporized during data collection.
c. The solute dissociated slightly in the solvent.
FREEZING PT DEPRESSION/MOLAR MASS OF A SOLID | Aug18
CHM 111/112
FORMAL LAB REPORTS – A GUIDELINE
This handout provides guidelines for writing a formal, typed laboratory report for CHM 111/112. These guidelines should
be similar (but not identical) to guidelines for reports in other Chemistry, Physics, or Natural Science courses.
General Tips:
•
Be concise. Say as much as is needed, while using as few words as possible (and still obeying common
grammatical rules). Lab reports should be thorough but not repetitious.
•
Write in the third person. Do not use words such as “I”, “we”, “I believe”, “in my opinion”, etc., especially
when referring to the experimental procedure or conclusions.
•
Use correct verb tenses.
1. The experimental procedure has already been conducted, so use third person, past tense, passive voice.
2. The Background and Discussion/Conclusion should be third person, present tense.
•
Take good notes, and write about what really happened, not what was supposed to happen.
•
Do not copy the lab handout – that is plagiarism! Use your own words to describe the experimental purpose
and background, the experimental procedure, your observations, and your results of the experiment. The lab
handout is to be used as a guide. If you really understand why you did what you did, your report can be
surprisingly concise.
Lab Report Format:
The following describes in detail the content of the sections required in the report. All sections are expected to be typed.
•
Title Page. Includes title of the experiment, student’s name, name of lab partner(s), date on which the experiment
was conducted.
•
Introduction. The introduction should include two main categories:
1) The Purpose or Objective of the experiment, expressed clearly in one or two sentences, including the main
method used to accomplish the purpose. Do not directly copy the “Objective” section from the handout,
since the handout often lists educational objectives in addition to the objectives for the experiment. Students
should only include the experimental objectives.
2) The Background or Theory of the experiment. This can include information from the introduction in the lab
handout, and should include any explanations of theories or equations used. Be concise, and include only the
background information that a reader would need to know in order to understand the purpose and methods of
the experiment. If useful, an example can be used to illustrate the theory.
•
Materials. A simple listing of the materials and equipment – it should be complete and accurate. Do not include
common supplies like disposable gloves, paper towels, weighing boats, balances.
•
Experimental Procedure. This section includes the process of the experiment as it was done in the laboratory.
It may be written out step-by-step in the form of a bulleted list. This section should be written in the passive
voice – it should NOT be a set of instructions. Use the experiment handout as a guide, but include just enough
information so that others who read the report would be able to duplicate the experiment at a later date. Do not
include any experimental results in the Procedure section.
•
Results. This section should include all results of the experiment, including:
1) Raw data – Masses, volumes, temperatures, etc., organized into tables or graphs when appropriate. Use the
lab handout as a guide in organizing the data.
2) Calculated results and/or important observations. Calculated results should be clearly displayed in a
table; qualitative results that do not fit in a table or graph may be written out in clear, concise sentences. Use
the lab handout as a guide to which results need to be reported. If required, averages and/or percent error
should be reported here.
3) Calculations. Usually only one sample calculation needs to be shown for each calculated result. Be sure to
use correct significant figures and correct units for all results and calculations. If required, the calculation for
average or percent error is shown here. If a graph is part of the lab, include it in this section.
•
Discussion. This is the section in which the student shows that he or she has a thorough understanding of the
concept of the experiment and the significance of the results obtained. The Discussion should be concise, and
may focus on some of the following areas (but not all areas):
a. Did the completed experiment accomplish the purpose as written in the Introduction? Provide one or two
sentences that summarize definitive conclusions from the results.
b. How did actual results compare with expected results (if known)?
c. An analysis of the experimental or percent error – comment on the magnitude of the error, and suggest
specific potential sources of error. Unless you noted an obvious equipment malfunction in your
Experimental Procedure section, do not list equipment malfunction or generic “human error” as
sources of error.
d. Give specific suggestions on how methods could be improved to minimize experimental error.
e. If assigned, any Post-Laboratory questions should be answered/incorporated into this section.
•
Conclusion. Write one to two sentences concisely stating what you determined in the experiment.
CHM 111-XXX Fall 20XX
Physical Properties of Pure Substances and Mixtures
Your Name
Your Partner’s Name (if any)
Date that experiment was conducted
Introduction
The purpose of this experiment was to investigate the relationship between the mass percent and density of a
series of increasingly concentrated aqueous salt solutions (in Part A); and to determine the density of an
unknown pure metal sample using the concept of water displacement (in Part B). The use of density to calculate
other physical properties (e.g., thickness) of a metal sample was also investigated in Part C.
The density of an aqueous salt solution (e.g., CaCl2) is known to be directly proportional to the mass percent of
solute in solution. Consequently, if a series of mass percent/density data points is obtained experimentally for
an aqueous salt solution, then the graphical relationship between the mass percent of solute and density should
result in a straight line. By use of this proportionality, the mass percent of solute in an aqueous solution of
unknown concentration can be obtained from its density. (Possibly insert example graph here…)
When a non-reactive solid is completely submerged in a liquid solvent, it displaces a volume of liquid that is
equal to its own volume. Consequently, if the mass of the dry solid is known, the density of the solid can be
determined experimentally, simply by measuring the volume of liquid it displaces, since d = m/V. The density of
the solid can then be used to help identify the solid, by comparing its experimental density to published
densities of known metals. (Insert example problem here…)
Materials
Just list them. Don’t include routine items such as paper towels, weigh boats, analytical balance etc. Focus on
the specific items that are unique to the experiment.
Procedure
(Third person, past tense, passive voice! Use just enough detail that another student could replicate your
results.)
Part A – Determination of Densities of Calcium Chloride Solutions
1. A clean, dry 1.00 mL volumetric flask/stopper assembly was massed to the nearest 0.0001 g
2. The volumetric flask was rinsed and filled to mark with deionized water of known temperature; the filled
flask/stopper was massed to the nearest 0.0001 g, and the density of pure water was determined.
3. Previously-prepared aqueous calcium chloride solutions of known mass percent (10%, 20%, 25%, 30%,
40%, 50%) were obtained. The 1.00 mL volumetric flask was rinsed and filled to mark with each solution;
the filled flask/stopper assembly was massed to the nearest 0.0001 g, and the density of each solution
was determined. Clean disposable pipets were used with each new sample, in order to avoid cross
contamination of the experimental data or original sample bottles.
4. You get the idea…
Results – Part A
(Can be very similar to table/presentation in lab handout)
Volume of flask: 1.00 mL
Mass of Empty Volumetric flask/stopper: _____________
Pure
H2O
10%
20%
25%
30%
40%
50%
Unk
Mass of
flask/stopper/solution
Mass of solution
Density of solution
Code for Unknown Solution: XXX
Mass Percent of Unknown Solution: XXX
Sample Density Calculation: You must show the calculation, not just the result. No one trusts your math
unless you show it!
Graph on Page XX
Results – Parts B and C
(Can be very similar to table/presentation in lab handout)
% Error Calculation: You must show the calculations!
Foil Thickness Calculation:
Discussion
This experiment conclusively demonstrates that physical properties of mixtures and pure substances can be
used to help identify unknown solution concentrations (as in the case of the aqueous salt solution in Part A), or
even to identify an unknown substance (as in the case of the metal in Part B).
Although the exact concentration of the aqueous salt solution in Part A was not known, the graphical
relationship between mass percent and density strongly indicates that that the mass percent of the unknown
solution is XX% – see graph on page xx. Since the true concentration of the test solution was not known, no
error analysis could be performed; however, the R2 value of xx for the linear trend line suggests a high degree of
correlation between the observed densities and mass percent values for all solutions.
The results of Part B indicate that the identity of the metal is most likely XXXX. Error analysis reveals a percent
error of XX%. The unusually high error was affected by the fact that a small amount of water accidently
splashed out of the graduated cylinder during Trial 2. This type of error could be minimized if additional
experimental trials were completed and averaged. Additionally, use of a more precise graduated cylinder to
determine the volume of displaced water would probably increase the certainty of each volume measurement.
(Obviously, this is where you discuss anything specific that went wrong, and its effect upon your results.)
(If any Post-lab Questions are assigned, they can be incorporated into the Discussion section)
Conclusions
In Part A, the densities of solutions with known CaCl2 concentrations were used to create a standardization
curve. This curve was used to determine the concentration of an unknown CaCl2 solution, and solution XXX was
found it to be to be XXX% CaCl2.
In Part B the density of an unknown metal was determined, and used to identify that metal. Unknown metal
XXX was determined to be XXX.
In Part C the thickness of a metal sheet was determined using the mass of the metal sheet and the density of the
metal. The thickness of metal XXX was determined to be XXX.