Last modified in 8/09 by Gary Mines
Instructor Notes for CHM 121 Experiments
Exp 2. Paper Chromatography (Slowinski, 8th Ed.)
Instructor Notes
The Hg2+ is supposed to appear as a pale yellow-orange ring with a high (~ 1) Rf value (it actually appears
to have an Rf value that is greater than 1, which is bizarre to say the least).
Added in F’09:
In recent semesters, this band has not always been visible for many student groups (after a day or so, it
sometimes turns into a dark blue band, but for some students it turns white). Kayshaun will make up
fresh solutions of Hg2+ for the F’09 classes’ knowns [and hopefully, the unknowns as well] and I’ll report
later (with your help) on whether or not that solved the problem.
The Ag+ appears as a pale yellow spot with a very low Rf value (~0) and sometimes has not been
discernable. If students wait until the next lab period to read their chromatograms (they can leave them in
their lab drawers), the Ag+ shows up more clearly (spot darkens).
Make sure students do not take more than they need of each solution! (they need << 1 mL each!).
Two "Un." lanes are indicated in Figure 2.2, but no mention is made in the write-up of applying the
unknown mixture to two separate lanes. Some students mistakenly (but reasonably!) think that they are
going to use two different unknown mixtures--they should not. (I also find it strange that the term
"known" is used to describe the mixture of all five knowns; each lane with a given cation is a "known" as
well and I think it is good to point this out to students)
I usually use the time during the development of the chromatogram to do worksheets on lecture-related
material rather than to test the spot colors with the stain. Also, if time is running short, one need not wait
a full 75 minutes to get a decent separation (45 minutes is generally sufficient).
Exp 1. (Parts A, B, and C) Densities of Liquids and Solids
(Slowinski, 8th
Ed.)
Instructor Notes
Be sure that students properly recover, wash, and dry their metal unknowns. Remind students to return
the pycnometers (with stoppers) to the tray (not their drawer!)
Added in Fall ’09: We are now (F’10) using single solid metal pieces (cylinders) rather than small pellets
of metal this semester, as the metal pellets are difficult to “control” and dry. Further information on this
potential change will be provided at the Orientation Week meeting and/or a follow-up email.
STUDENTS MUST BE WARNED ABOUT ADDING THE METAL CYLINDER GENTLY TO THE
FLASK (RATHER THAN “DROPPING” IT IN) TO AVOID GLASS BREAKAGE.
The air spigots do work in our lab at DP, but only if the master switch is turned on (located on the wall
just to the left of the stockroom entrance as you face it from the lab). CAUTION: Spigots should
NEVER be turned "fully on" as the pressure will be dangerously high; have students turn the spigot very
carefully so that it is just barely on. N.B. Sometimes condensed water (!) is present in the air lines, so
students should test them out first before using them to dry flasks. Note: Air is always on at RHC.
The manual often states to measure mass "to the nearest milligram". Our balances measure out to the
next decimal place (0.1 mg) and I don't see why they should limit the precision in the measurements
arbitrarily. That being said, in part B, for the more volatile unknown liquids, many students report that
the mass is constantly decreasing (in the last decimal place on the balance) after they've filled the flask
up. Whether this is due to inadequate drying or to poor matching of stoppers with flasks is not certain,
but you may wish to have students round to the nearest milligram in these cases, should problems arise.
Exp 6. (Parts A and D) Properties of Hydrates
(Slowinski, 8th Ed.)
Instructor Notes
Make sure students do NOT use a plastic test tube rack (they do melt!).
Stress that only a gentle heating should be done in Part A! If students overheat (or heat do dryness), the
tubes are sometimes impossible to clean.
Although the manual says to use "small" test tubes, I have students use the medium-sized tubes since they
are easier to clean (see above).
Students often have trouble making the proper conclusion regarding "water soluble" in Part A. For one
thing, several of the solids in this experiment are slow to dissolve and the amounts they use sometimes
don't dissolve completely in just a few cm3 of water. Also, many students don't attempt to swirl, vortex,
or stir the solution--they expect it to just "disappear" (seemingly right away!) if it is soluble. Lastly, some
students confuse "homogeneous" with "soluble" (esp. with CaCO3; the shaken mixture looks "white like
milk" so some mistakenly conclude that it is soluble). Be on the look-out for these potential problems!
The data table for Part A should really include columns for "Color of solid (before heating)" and "Color
of Solution (if residue redissolves)" because a property of hydrates mentioned in the write-up is that if
their anhydrous salts dissolve, the "solutions have a color similar to the (original) solid hydrate". I tell
students to add those columns to the table (they can squeeze it in!). You must also tell them, however,
that "white" hydrates will not give a "white" solution upon dissolution—it will be "colorless".
NEW in F’09: For Part D: Samples of CaSO4 seem to have consistently given a mass percent of about
19.7% in recent semesters when theory says it should be 20.93%. A new bottle in F’09 will be ordered,
and I will let folks know what I see (please feel free to send me your students’ data as well).
UPDATE in F’10: The new bottle purchased in F’09 did not give better data, but another new bottle
purchased in S’10 (from a different company) did give much better results. So Kayshaun (and Jose)
should be purchasing CuSO4 from that company from here on out. Hopefully we’ll see consistently good
results.
ALSO, I have had some success using CoCl2 hydrate in Part A in place of one of the white compounds. I
did not like the fact that a lot was made about color changes in the discussion of this experiment, but then
only one of six compounds in Part A was colored. We may end up making this change in all sections.
Exp 4. Determination of a Chemical Formula
(Slowinski, 8th Ed.)
Instructor Notes
Show students how to pick up the Bunsen burner and move the flame around to gently and
evenly heat; if they leave the burner in one place, it is less likely that they will evenly heat the
solid.
Be prepared to hear many crucible lids fall! Try to warn them to be careful of this and show
them how to place the lid properly ajar with some support. Check to see that the crucible and
clay triangle are a good size match—a large crucible will tip over easily in a small triangle and
occasionally a small crucible can fall through a triangle that has been "widened" to fit a larger
crucible.
Many students want to use a microspatula to mix up the brown crystals to see if the inside has
turned color. I discourage this due to the transfer of material to the spatula.
Be sure to instruct students to clamp the filter flask to a ring stand before use.
Remind them (show them?) that balances will read inaccurately if items are not at room
temperature.
A drying oven can be used to help dry the copper metal (New in F’09: we are currently looking
into the possibility of buying some heat lamps to see if that works better This has not been
pursued thus far]). Tell students to put their filter paper with copper onto a watch glass for easier
handling, and make sure they initial the watch glass or otherwise remember where they put their
sample in the oven so they don't get mixed up. Alternatively, students can store their copper in
their drawers and let it air dry over the week, if you don't mind them turning in their lab report
the following week.
Exp 24. (A and B only): The Standardization of a Basic Solution
and the Determination of the Molar Mass of an Acid (Slowinski, 8th Ed.)
Variation in procedure from lab manual: Students will NOT prepare their own NaOH(aq)
solutions. Thus 6 M NaOH(aq) and Florence flasks will not be needed. A large volume of dilute
NaOH(aq) will be provided.
Unknowns: Change in procedure from lab manual: Each pair will be given a vial with a lot
more solid than they will actually use. Instructors should tell students to use ~0.5 to 0.6 g of
solid per trial (no matter which unknown they have) and that they should expect to have a
lot of solid remaining in their vial. (Their manual will tell them to use “half their sample”,
which would be a catastrophe!) See table below for theoretical titration volumes for 0.6 g of
each of the four unknowns:
Unknown
mandelic acid
potassium hydrogen phthalate
sodium dihydrogen phosphate
monohydrate (NaH2PO4∙H2O)
sodium hydrogen sulfate
monohydrate (NaHSO4∙H2O)
sodium hydrogen tartrate*
Molar Mass
(g/mol)
152.1
204.2
mL of 0.1 M NaOH
needed to titrate 0.6 g
39.4 mL
29.4 mL
138.0
43.5 mL
138.1
43.4 mL
172.1
34.9 mL
*NOTE: As of F’10 (if not S’10?), unknowns that were previously glycolic acid (MM = 76.05 g/mol)
have been replaced by sodium hydrogen tartrate (MM = 172.1 g/mol). This has worked well in trials in
S’10 and gives everyone 5 “good” options now.
Exp 30. Determination of Iron by Reaction with Permanganate: A
Redox Titration (Slowinski, 8th Ed.)
(for a class of 24 students working in pairs)
Change in procedure: Students will be provided with 1 M H2SO4--they will not need to make
the dilution.
You will need to supply the balanced net ionic equation to students for this experiment (unless you teach
them how to balance redox equations in CHM 121, which I don't do until CHM 122). It is (w/o state
designations): 8 H+ + MnO4- + 5 Fe2+  Mn2+ + 5 Fe3+ + 4 H2O
Tell students to use about 1.3 to 1.5 g rather than 1.0 g of solid; otherwise volumes of titrant needed
will be too small. With 1.5 g of solid, a 13.1% Fe sample should take about 47 mL of 0.015 M MnO4-(aq)
to reach equivalence, and a 5.6% Fe sample should take about 20 mL.
NOTE: Over the past few semesters students in my section have consistently gotten lower than 13.11 %
for the 13.11% sample (range from 11.7 – 12.6%). Since the number of data points is low, please send
your students' results to me so I can see if that pattern continues.
Exp 8. Verifying the
Absolute Zero of Temperature (Slowinski, 8th Ed.)
We are currently looking into changes to make this better (maybe stabilize the U-tube before giving to
students). As of F’09, a one-page summary “procedure” list has been made to try to make this experiment
work better for all students and faculty. Going through this step-by-step with the students (while you
actually show them with a setup at the instructor bench) has helped out greatly in my experience.
Handout Lab--The Air Bag Problem
Added in F’09:
Since no specific procedure is given in the handout for how to set up the bag, I generally just
demo this for the class during prelab. The procedure is as follows: First add the solid (NaHCO3)
to the bag, collect it in one corner, and then grab the corner and twist it a couple of times to
separate that region of the bag from the rest of the interior space of the bag. Rotate the bag 45
degrees so that the other corner is lower (and the corner with the solid is higher). Carefully pour
the solution of H2SO4(aq) into the lower (and empty) corner so that it doesn’t touch the corner
with the solid. Push out as much of the air in the bag as is reasonable, and seal the bag. Unravel
the corner with the solid and rock the bag back and forth to mix the acid and solid and generate
the CO2 gas.
Although no procedure for determining the volume of the filled bag is given, one or two large
(2000 mL?) plastic graduated cylinders are set out on the tray for the experiment, and most
groups figure out that they can get an estimate for the volume of the bag by filling up the bag
with water and then pouring the water into the large graduated cylinder.
Experiment 14 A, B and C. Heat Effects and Calorimetry
I have found that if stirring is done quickly using the digital thermometer probe, no cap or stirrer
is needed. The faster mixing perhaps makes up for the lack of a top.
Added in F’09: We will likely try using single solid cylinders of metal in Part A rather than little
“pellets” (as noted above for the density experiment). I have tried this with great success [within
less than 1 % error] (myself, as a demo) using a copper cylinder and putting it in one of the large
test tubes in the student drawers (not the extra large “ignition tubes” that have been called for by
the lab manual and which have been put out in the past). You can still try using the ignition tube
if you like, but I think using a smaller tube will help quicken the temperature equilibration time.
It is unclear whether or not 10 minutes (minimum time mentioned in lab manual) is sufficient to
temperature equilibrate, as I’m certain I let mine sit much longer than that when I did my demo.
You may wish to have students set this up first but then do Part B first and come back to Part A
afterward.
NOTE in F’10: I want to remind faculty to tell students to use their own “large” test tubes
instead of the ignition tubes for Part A now that cylinders are being used. Also, let them
equilibrate the setup for longer than 10 minutes (perhaps have them do Part B and / or C while
Part A is temperature-equilibrating) for best results.
Exp 12. Alkaline Earths and Halogens
**Stock room note: Make sure that the hexane dropper bottles used for this experiment are NOT the
same ones as are used in the organic lab; on 10/17/06, a bleaching of Br2 color was observed and traced to
a contaminated hexanes bottle. [This has not been a problem as of late.]
**Also try to figure out how to deal with the "degradation of stoppers" issue with Cl2 and Br2. Cracked
stopper caused a leakage on 10/17/06. [This has not been a problem as of late.]
Added in F’11: Regarding the problem noted in “F’09 below”, Roy has had a simple solution that I want
to share: Simply direct students to add 2 mL instead of 1 mL of the NaI(aq) to the Br2 tube. This will
solve the problem of “excess Br2”.
Added in S’10: Put all halogen / halide /hexane bottles in a hood so that students won’t need to breathe
the harmful vapors. Since we have two sets of bottles, I suggest putting the two sets in two separate
hoods.
Added in F’09: Note: If the halogen solutions are too old, the colors are not deep enough and incorrect
observations may occur. So we recently tried (at DP) making the solutions within a day or two of the
experiment. However, preparing the solutions fresh has been found to cause the following unanticipated
problem: If the bromine solution is too concentrated, then the tube in which aqueous bromine is mixed
with iodide, there appears to be excess bromine, making the hexane layer appear dark enough (and red
enough) for students to conclude that there is “bromine present” and thus there is “no reaction” (even
though a trained eye can tell that the color has some purple iodine in there). We are still working on
determining the right age” of these solutions to have the optimum chance for correct student observations.
If you think a solution is either too dilute or too concentrated, please contact a stockroom staff person.
Handout Lab --What’s In These Bottles?
See the separate handout for instructor notes. MAKE SURE NOT TO GIVE STUDENTS (or
post) THE PAGE OF THAT DOCUMENT THAT HAS THE INSTRUCTOR INFORMATION
(KEY) ON IT!! [This is no longer a problem since I’ve separated the key from the handout
document. The key is in the “Keys” document now.]
Experiment 13. The Geometrical Structure of Molecules
This lab activity is designed to be done following discussion of Lewis dot structures.
Experiment 17. Classification of Chemical Substances
Students will check for conductivity rather than resistivity, as mentioned in the manual.
NEW in Fall ’08: An insulated pliers, O-ring, and pennies will be provided at DP on request in
Fall ’08 as a test trial. At least one instructor tried this in either Spring or Summer, although I do
not know who it was or how it turned out. Please let me know if you try anything you think I
might not be aware of!
If students do not test conductivity in powders, they cannot distinguish between covalent
network solids and powdered metals (of high melting point). In any case, they cannot
unambiguously distinguish between insoluble ionic compounds of high melting point from
covalent network solids.
New in F’09: There has been a request to use a solid metal rather than a powdered metal, and to
avoid using silicon as an unknown. If you have such a request, let me know ahead of time and
we’ll see if it can be arranged. A “global” change in the unknowns for this experiment is being
considered, but has not been decided on at this time.
Experiment 19 (Parts A and B only)
Molar Mass Determination by Depression of the Freezing Point
 Ice can be found from the ice machine in the lab. An ice-crusher is now available if
students (or you) would like to provide crushed ice, for ease of stirring and less spillage.
 Students must stir solutions quite rapidly and for longer than they expect in order to get the
greater than 4-degree depression mentioned in the manual.* [Some students worry that if
too much of the ice melts, they will get the wrong answer. In fact, the ice must melt for the
temperature to decrease (it is the endothermic process of melting that absorbs the thermal
energy in the solution and results in lowering the temperature).]
Added in F’09:
* Perhaps even more importantly, students will also get a smaller than expected decrease in
freezing point if they use too much water to begin with. Although they are not supposed
to measure out a specific amount of water, the instructions say to use “more ice than
water” (in Part A). Students need to be reminded of this. Furthermore, although it is not
clear on a first reading, the exact same cup of ice water that is used in Part A is used for
Part B. Thus, you need to tell students to only fill the cup (in Part A) about halfway or
else 1) there will not be any “room” to dilute the solution for the second
measurement, and 2) the fp depression will be too small (because there will be too
much water)
 THIS EXPERIMENT MUST BE READ VERY CAREFULLY TO AVOID PITFALLS!
For instance, many students do not realize that they are not supposed to measure out a
specific amount of water and ice for Part B. In fact, they are supposed to use the exact
same cup/mixture as was used in Part A. Also, the lab manual does not actually ask
students to do a second "trial"--it says to do a second "measurement". They should not
dump out their solution after filtering out the ice! They use that same solution, after
dilution with more ice and water, for the second "measurement". Thus students should not
use any more unknown for their second "measurement". Lastly, students often do not
understand what they are supposed to do with respect to using the (hypothetical) values
given to "estimate" the amount of unknown liquid to use in Part B. That is, they are never
told how much to use (unless you decide to directly tell them).