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Lab 3

QUESTIONS

Remember that molality (m) = moles of solute / kg solvent. So, using your data from Table 1, calculate the moles of each solute (glycerol, NaCl, and CaCl2) used in this lab by using the experimental molality and the mass (in kg) of water in each trial.. You must show all work to receive credit.

Recall that the molar mass of a substance is expressed in g/mol. If we have a mass of a substance, and we know how many moles of it we have in that given mass, we can divide g/mol to determine the molar mass. So, using your experimental data from Table 1 and your answer to the previous question, calculate the molar mass for each solute. You must show all work to receive credit.

Calculate the percent error for your experimental values for each solute. Is your percent error value relatively constant with each trial? If so, what might this indicate (include accuracy and precision as part of your answer)? You must show all work to receive credit.

What are some possible sources of error in your experiment? Discuss at least 3 in detail.

Which solution of salt (NaCl or CaCl2) may be better to salt the roads? Why? (HINT: Don’t just focus on the final freezing point in each case—consider all your data and how it may be tied to efficiency and economic factors.)

Lab 3: Molar Mass and Freezing Point Depression

OBJECTIVE:

The objective of this experiment is to determine the molar mass of a known substance using its freezing point

depression in a solution. You will be given three solutions in addition to the standard solution (water). The freezing

point of water will be determined and then the freezing points of three other solutions will be determined. These
solutions used in this lab are composed of sodium chloride (NaCl), calcium chloride (CaCl2), and glycerol (C3H8O3). The

freezing point data will be used to calculate the molar mass.

MATERIALS:

Vernier temperature probe

Vernier LabQuest 2 interface

PROCEDURE

1. Put on your goggles and lab apron.

2. Label one weigh boat as NaCl and the other CaCl2.

3. Turn on your LabQuest2 interface, and while it is booting up, proceed to the next step.

4. Assemble a freezing point apparatus by filling a 600 mL beaker with crushed ice (the smaller the ice pieces,

the better). You will have to use small pieces of ice for this lab to work properly, so that may require you to

physically crush your ice. You will be adding more ice as it melts as the experiment goes along, so if you have
someone who can be your “Iceman” while you run “Maverick” (look it up if you don’t get the reference ☺),

that may make things easier for you. Otherwise, prepare accordingly and have crushed ice ready.

5. Attach a utility clamp to your ring stand and position it above the ice bath as shown. Note that the ice in this

picture is not crushed, but yours needs to be:

6. Connect your temperature probe to your LabQuest 2 interface CH 1 port and make sure it is working

(showing a readout) before moving to the next step.

7. Place the NaCl weigh boat on your scale and tare (re-zero) the scale. Then add about 35.0 g of NaCl to the

weigh boat. This does not have to be exact. If you would rather hold onto your NaCl from your kit (you know,
just in case!), you can just use regular table salt from the kitchen or supply room. It doesn’t have to be pure

NaCl, so it’s fine. Pour this NaCl into your ice bath and stir as well as possible. Recall that by adding solute to a

solvent, you lower the solvent’s freezing point. We are not measuring the freezing point change of this ice

bath (which is why pure NaCl is not necessary), but we are going to measure the freezing point depression for

other solutions. We are adding the NaCl in this case so that we can make the liquid in the ice bath cold—

colder than we could if it were just pure water.

8. Place your temperature probe in the center of the ice “bath” (don’t let it touch the bottom or sides of the

beaker) and monitor the temperature while you continue to work. Once the temperature reaches about -10
oC, your ice bath is ready. If it goes colder, that’s OK. This will take a couple minutes. While you wait, proceed

with the next step.

9. Place the NaCl weigh boat on your scale and tare (re-zero) the scale. Then add 2.0 g of pure NaCl (this must

be from your kit) to the weigh boat. Do your best to be as accurate as possible and record your mass in Table

1 (see below). Carefully set your sample aside (don’t lose any of your sample by bumping it, etc.).

10. Place the CaCl2 weigh boat on your scale and tare (re-zero) the scale if necessary. Then add 2.0 g of CaCl2 to
the weigh boat. Do your best to be as accurate as possible and record the mass in Table 1 (see below).
Carefully set your sample aside (don’t lose any of your sample by bumping it, etc.).

11. Label 4 test tubes #1, #2, #3, and #4.

12. Using your 100 mL graduated cylinder, measure out 15.0 mL of distilled water (dH2O) and pour it into test

tube #1.

13. When the ice bath is ready, remove the thermometer and rinse it with distilled water. Insert the

thermometer through the hole of a 1-hole rubber stopper and slide the stopper up until it touches the handle

on the probe. You can drop a little dH2O in the hole for lubrication if needed and use a bit of a twisting
motion to “screw” on the stopper as push it up the probe. Insert this into the top of test tube #1 containing

the dH2O to create a sealed system. The stainless-steel probe should be in the water, but not touching any

part of the test tube.

14. Tap the gray box that contains the headings “Mode,” “Rate,” and “Duration” that is located next to the red
box containing the temperature reading. Change the Duration to 30 min and the rate to 120 samples/min.

Tap OK. It won’t take 30 minutes to get your sample, but we are being safe so that the data collection doesn’t

automatically shut off.

15. Tap the graph icon on your LabQuest2 interface to display the graphing screen. Tap Graph at the top, then

tap Graph Options. Change the y-axis top value to 15, and the bottom value to -15. Click OK.

16. Suspend test tube #1 with attached thermometer in the ice bath using the utility clamp. Allow the test tube

to suspend as fully into the ice bath as possible, but don’t let it touch the bottom or sides of the ice bath

beaker. Keep the crushed ice packed around the test tube. If some of it is melting, it is OK—don’t dump it out

or you will remove some of the salt you added. Just keep adding ice as needed.

17. Wait for the temperature to drop to about 15 oC, then tap the Collect button to begin data collection.

Watch your graph to visually observe the freezing point. It will be the area on the graph where instead of

dropping, the data reaches a plateau and the temperature levels off. The temperature during this time seems

relatively constant, since once the freezing point is reached, all additional energy removal from the system
works toward completely reaching the solid state. Only after the solid state has been fully reached

throughout the sample would you see the temperature begin to drop again. This second drop won’t be visible
in this lab due to limitations in our instrumentation and setup.

18. While your experiment is running, take a picture of you with your setup and data collection process. The

complete setup (ring stand, clamp, ice bath, utility clamp, test tube #1 with temperature probe attached,

etc.) must be visible, and your LabQuest2 data collection screen must be visible as well so that your data

collection progress can be seen in the picture. The live graphing process should be clearly visible on the

interface screen. Your face must be visible in the picture also, as well as a label with your name/student ID,

the lab title, and the date. Unless all four—your face, name/ID, the lab title, and the date are clear, you will

not receive credit. Save this picture in a PDF document in order to submit it in the Lab 3 Assignment.

19. Once you see the freezing point occur, and the graph remains “flat” for several minutes, tap the Stop

button to stop data collection (or you can wait the full 30 minutes to be certain if you are looking to hang out

and kill some time). In the dH2O sample, you may witness “super cooling,” where the graph drops so quickly

it ends up shooting past the freezing point, and then, instead of slowly leveling off, spikes back up briefly to

the actual freezing point. If you witness this, it is normal. Just pay attention to the “flat” line data to the right

of that spike.

20. The freezing temperature can be determined by finding the mean temperature in the portion of the graph

with nearly constant temperature. Select the data points (touch and drag) on the graph the mark as closely as
possible the beginning and end (left and right sides) of this “flat” area. Be sure to only include data that is on

that plateau, and not any data points that are part of the sharp decreasing trend. This section may not be
perfectly “flat,” in which case you will have to make a judgement call regarding what points to include.

21. Tap Analyze at the top of the screen, then tap Statistics. Check the box for the data set in question (in this

case, Temperature). You should now see the statistical information displayed in a panel to the right of your

graph. Record the mean temperature value in Table 1 (see below). This is the freezing point for dH2O under

your laboratory conditions.

22. Remove your temperature probe (with 1-hole stopper) from the test tube, rinse it with dH2O and set aside.

23. Remove your test tube #1 from the ice bath and set it aside (in your test tube rack).

24. Make sure there is still enough crushed ice in your bath—if not, add some more. Stir the ice.

25. Use your 10 mL graduated cylinder to measure out 5.0 mL of glycerol and pour it into a 100 mL beaker. Your

vial containing the glycerol may say already that the contents are 5 mL, but you need to be sure. You may

have to cut off the tip off the bottle to get the glycerol to come out.

26. Use your 100 mL graduated cylinder (be sure you rinse with dH2O before using) to measure 25.0 mL of dH2O.
Transfer part of this 25.0 mL into your 10.0 mL graduated cylinder to help rinse out any residual glycerol, then

dump this water/glycerol into the 100 mL beaker containing the glycerol and stir with a stirring rod. Repeat

this “rinsing” until you get as much of the remaining glycerol from the 10.0 mL graduated cylinder as possible.
Pour this thoroughly stirred glycerol solution from the 100 mL beaker into test tube #2.

27. Tap the File Cabinet icon above the temperature readout to begin a new data run (Run 2). This will create a
new data run (Run 2), and your previous data from test tube #1 will not be erased. It is still stored in the

interface.

https://edge.apus.edu/access/content/group/science-and-technology-common/CHEM/CHEM133/Labs/How_to_Upload_PDF_Pictures

https://edge.apus.edu/access/content/group/science-and-technology-common/CHEM/CHEM133/Labs/How_to_Upload_PDF_Pictures

28. Insert the thermometer with 1-hole stopper into test tube #2 as you did before, suspend it in the ice bath as

you did before, and click Collect to begin data collection. Refill and repack your crushed ice around

the test tube as needed. After the freezing point can be seen like before, tap the Stop button to stop
data collection. If the temperature is still dropping (it will drop more slowly, probably, than it did with just
dH2O), don’t stop collecting data. Determine the freezing temperature by analyzing the data to determine the

mean temperature of the “flat” portion as you did above. This is the freezing point of glycerol under your

laboratory conditions. Record it in Table 1 (see below).

29. Disassemble as you did in steps 22-24 above.

30. Rinse out the 100 mL beaker well with dH2O. Using your 100 mL graduated cylinder (again, after dH2O rinse)

measure out 25.0 mL of dH2O and pour it into the 100 mL beaker.

31. Add to this beaker your 2.0 g of pure NaCl you measured out in step 9 above and stir to dissolve (be patient
and stir—warm it a bit if necessary). Pour this NaCl solution into test tube #3 and determine its freezing point
as you did with dH2O and glycerol (be sure to tap the File Cabinet icon before you start to begin Run 3).

32. Repeat steps 29-31 with your 2.0 g of CaCl2 in test tube #4 (Run 4).

33. Make a data table in either Word or Excel showing the freezing point and freezing point depression of each

solution, the associated van’t Hoff factor (i) for each solute, the molality (m) of each solution, the mass of the

solute used for each solution, and the mass of the solvent (H2O) in each solution. Remember that these are
all water-based solutions (water is the solvent), so any freezing point depression is relative to whatever your

dH2O (test tube #1) freezing point is. In other words, when considering Δt (using Ti – Tf), test tube #1 (dH2O)

represents your Ti in each case. Your table should look something like this:

Table 1: Freezing Point Data

Solution Freezing Point (oC) Δt (Ti – Tf) i Molality (m) Solute Mass (g) Solvent Mass (kg)

H2O

Glycerol

NaCl

CaCl2

Note: Kf = 1.86 oC•kg/mol; Δt = iKfm, therefore m = Δt/iKf; d = 1.00 g/mL for H2O; d = 1.26 g/mL for glycerol

Calculations:

(show all your work)

HINT: In the above table, the Freezing Point column is your measured freezing points in this lab, and the Δt

column is the difference in freezing points between your 3 solutions (test tubes #2-4) and pure water (test

tube #1). The column “i” applies to electrolyte (ionic) solutions and represents the ratio of moles of particles in

solution to moles of formula units dissolved. In this case, glycerol has a value of 1 (it does not dissociate into

separate units), NaCl has a value of 2 (it separates into 2 units—1 Na+ and 1 Cl-), and CaCl2 has a value of 3 (it

separates into 3 units—1 Ca2+ and 2 Cl-). The molality column is experimentally determined by solving the

equation Δt = iKfm for m. Solute mass is the amount of solute that was weighed out to make the solution—in

the case of glycerol, since it was a liquid, you can use its density along with the volume of it you used to find

the mass. Solvent mass in grams is simply the same value as the volume is in mL, since the solvent in each

case is water, and its density is 1.00 g/mL. Simply convert from g to kg for this column.

34. Save your Table 1 as a PDF file to be submitted with the Lab 3 assignment. Be sure your document includes

your name/student ID, lab title, and date. Unless all three—your name/student ID, the lab title, and the date
are on the document and legible, you will not receive credit. All columns and values must be clearly labeled,

and a calculations section below your table must clearly and neatly show how you determined your values.

These calculations must be in order as they appear on the table as well to receive

credit.

35. Tap the data table tab to enter the area containing the data tables of all your data runs. Tap the titles above

each table and change the names to more clearly label the data. For example, instead of Run 1, type in H2O,

and then C3H8O3 for Run 2, NaCl for Run 3, and CaCl2 for Run 4, or otherwise according to their respective

table if you messed up the order for some reason. Then tap the Graphing tab again to return to the graphing

screen, and make sure these new titles are reflected (tap the box to the left of the file cabinet where your
Run options used to be to make sure the runs are now labeled with the correct chemical name).

36. Create PDF documents of the graph from each data run—one for each test tube (#1-4). Be sure the statistics

are included on each graph so that the mean temperature value is indicated. Follow the exact same

procedure for creating this PDF as was outline in Lab 2: General Chemistry Laboratory Safety and Vernier
Intro, steps 31-33. If you do not submit the graphs in the exact same manner as in Lab 2, you will not receive

credit.

37. Create data table files of the data from each data run—one for each test tube (#1-4). Follow the exact same

procedure for creating this text file as was outline in Lab 2: General Chemistry Laboratory Safety and Vernier

Intro, steps 34-35. If you do not submit the tables in the exact same manner as in Lab 2, you will not receive

credit.

38. You should have 10 files to submit with this lab = 1 PDF of a pic of your lab setup + Table 1 + 4 graphs + 4

data tables. All pictures, graphs, tables, and labels must be clear, or they will not receive credit.

39. Once you are sure you have all the files you need, clean up your lab and put away your materials.

40. Tap the Home icon on your LabQuest2 interface, then tap System, then tap Shut Down. This will safely power

down the system, but it will erase your data (unless you want to save it). So, make sure you have emailed

yourself your graphs and tables before performing this step!

41. Wash your glassware (and plasticware) with warm water and soap. To minimize spotting and mineral

contamination, it is best to do a final rinse with dH2O. Dry your glassware (or leave set to dry) and put it away.

All chemicals used in this lab can be safely washed down the sink.

42. Answer the Post-Lab Questions below.

43. Go back into the course classroom, click on Tests & Quizzes located on the left side menu, and complete the

Lab 3 Assignment posted there. Remember, it is in a quiz format, but it is not a quiz. You can access it as

many times as you wish while entering your answers, and it is not timed. Just be sure that you do not actually

click “Submit” until you are ready to be graded.

Post-Lab Questions

1. Remember that molality (m) = moles of solute / kg solvent. So, using your data from Table 1, calculate the

moles of each solute (glycerol, NaCl, and CaCl2) used in this lab by using the experimental molality and the

mass (in kg) of water in each trial.

2. Recall that the molar mass of a substance is expressed in g/mol. If we have a mass of a substance, and we

know how many moles of it we have in that given mass, we can divide g/mol to determine the molar mass.

So, using your experimental data from Table 1 and your answer to the previous question, calculate the molar

mass for each solute.

3. Use the periodic table to determine the theoretical molar mass for each solute.

4. Calculate the percent error for your experimental values for each solute. Is your percent error value relatively

constant with each trial? If so, what might this indicate?

5. What are some possible sources of error in your experiment? Discuss in detail.

6. Which solution of salt (NaCl or CaCl2) may be better to salt the roads? Why? (HINT: Don’t just focus on the
final freezing point in each case—consider all your data and how it may be tied to efficiency and economic
factors.)