A post lab discussion on paper chromatography of amino acids.
Post-Laboratory Discussion Guidance and Rubric
What is a postlab discussion?
A postlab discussion is a short summary written in paragraph style of what occurred in lab and a short analysis of
results. The discussions that we do are in place of a full report, since it would be excessive to write a full report for each
lab experiment. You will gain similar writing skills from writing postlab discussions as you would writing full lab reports
without the time commitment since full lab reports have a significant amount of redundant text and boilerplate.
The requirements for a postlab discussion are actually quite the opposite from typical reports/essays, as the goal
is be short and concise. The requirements are that you convey the required information in under a certain number of
sentences. A postlab discussion should be under 1 page (1 page max) and should not contain redundant or flowery text,
but instead be in a simple, technical style. Avoid the use of personal pronouns to keep an impersonal perspective.
Postlab discussions should also be for someone unfamiliar with the exact details of the experiment.
A good discussion is a summary of the goals and results of an experiment. Upon reading the discussion, a person
should be able to answer: Why, How, What Happened, and What’s Next regarding the experiment performed. Each of
these points will be expanded below.
What should a postlab discussion contain?
1. Objective: The first sentence of the discussion should explain WHY the experiment was performed. Call it a
Purpose Statement, or Objective. Maximum, two sentences. Make sure you identify the specific chemicals that
are the focus of the experiment even if unknown, for example “Unknown A”.
2. Procedural Outline: Briefly summarize the methods used for the experiment. Limit to 2 or 4 sentences, report
exact substances used in the experiment but not data. Procedure must be written in 3rd person passive voice,
past tense and contain important information but not unnecessary steps or details. Any special laboratory
equipment used should be noted (most often this will be a Vernier device and LoggerPro software). You typically
should not mention what glassware you use unless it is actually important to the method. This does not need to
be sufficient detail for someone to repeat to the experiment.
3. Results: Restate the relevant results. Relevant results will support the discussion of results. This could be the
average value obtained for a measurement along with its percent error and standard deviation. In some labs, it
could be a percent recovery or percent yield, or if the objective of the lab is to make or observe something, it
could just be reactions and/or observations.
• A Table may be used if there are many numbers, but usually this is not the case. You only need to report
relevant results, so this will not be very many numbers. If you find yourself wanting to make a table,
think are all of the numbers relevant? If you just dump a table full of numbers that are not important,
you will get points taken off.
4. Brief Discussion: Discuss if the objective was met and if results of the experiment met expectations (are they
results reasonable). The expectations are not “It met my expectations” but are the results consistent with
methods used. Don’t just say the results “met expectations.” If the results are quantitatively comparable to
other sources, make a quantitative comparison using the standard deviation or percent error of results to the
percent precision of the equipment used. If the results are not quantitatively comparable (like with qualitative
data like color, smell, texture, etc), you will make your best judgment.
• If results are off from expected, then suggest a logical reason why that is the case.
• Suggest specific sources of error – if needed to account for error.
o Unacceptable sources of error include:
❖ Calculation error, Measurement error, Human error
5. Compare your work to theoretical/literature values and to the work of others in lab.
The works of others will always include other lab groups, as this will give you more confidence/doubt in
your results. Include the names of the other lab group(s) of whose data you compared. Picking another
group to compare data should be done with consideration, since it can distinguish between a poorly
executed method and an improper method. Pick a group that seems to have good results, and compare
o Don’t just dump their (average) result into your report; you can give it but it is typically better
to just give how far off your (average) result was. For instance “Jack and Jill determined the
boiling point of ethanol to be 77.8 °C which is 0.8% difference from this experiment and is
within the error of the temperature probe”
• When possible to compare to literature sources, include a citation (do not use Wikipedia). When
comparing a numerical value, you should always include the percent error between your numbers and
the literature value. Sometimes comparison to literature is not a number but a quality or observation.
Only very rarely are there no theoretical comparisons.
6. Suggest how this experiment could be improved upon, and also other application of it.
• You must give an improvement. This will get you to think. You may suggest a change in the procedure
where a better method could be used. Or you could suggest a different tool to use. Or even a different
experimental setup for doing the same thing.
o Unacceptable improvements include:
❖ Using new equipment, having more time to do the lab, having more clear instructions,
being more careful when making measurements
❖ Having something that would make things easier for you to understand the lab
❖ Having new equipment
o Improvements you have to be careful with
❖ Using more accurate glassware/equipment – you could say that for anything. However
often we use lower precision tools for speed and more accurate tools for what is
important. So you would actually have to pick a situation that mattered.
❖ Having something that makes it easier for you to do the lab. If it is something that would
reduce the amount of error that is fine, but an improvement for simple convenience is
a. example: if there was a device that would hold the beaker.
Unacceptable [that device is called your hand]
b. example: if there was a clamp to hold the tube so as to help prevent movements
that cause leaks.
• You must think of another application of the same or similar method to achieve a different objective.
This will get you to think. You cannot say that the method could be used with “other chemicals”. That is
not getting you to think. You need to think of a specific chemical, or better yet, come up with a similar
situation in which the method(s) could be used.
o Unacceptable applications include
❖ Vague applications—“other chemicals”
❖ Applications of curiosity – “add Kool-aid to the mix and see if the results are the
• Note you must have one of each (improvement and application). Including two of one and none of the
other will result in half credit.
Some examples of sections for 2 different postlabs (NOTE this has been broken up
into sections, but that is not the format for a postlab discussion.)
1. “A relative viscosity scale was established, then use this scale to determine the relative viscosity of an unknown
2. “The quantity of iron in a vitamin tablet was determined by colorimetry.”
◼ Notice the examples above are concise – they state WHAT was determined and the method used. Many times a
similar statement can be found in the laboratory handout at the end of the introduction. Take care not to
plagiarize the lab handouts
1. “A relative viscosity scale was developed by preparing a series of starch solutions of decreasing concentration,
and measuring the time it took for a marble to travel through large test tube containing each solution. Relative
viscosity for each solution was then estimated by dividing all marble travel times by the shortest value. The
relative viscosities of an unknown substance was then measured and estimated based on the scale.”
2. “A vitamin tablet containing iron was then crushed, dissolved in acid, then diluted and its absorbance was
measured. A calibration curve for iron analysis was prepared by measuring the absorbance of a series of iron
solutions of known concentrations using a Vernier colorimeter. Using the measured absorbance and the linear
equation of the calibration curve, the quantity of iron in the iron tablet was determined”
1. “the relative viscosity of Unknown Solution D was found to be 2.68, with a percent error of 12.6% with the
closest match of oleic acid.”
2. “Using the calibration curve which yielded a linear fit of y = 3025x and R2 of 0.9832, the experimentally
determined mass of iron in the tablet is 0.0345 grams per tablet, with a percent error of 1.47% from the 34 mg
listed on the bottle.”
Brief Discussion and Comparison (they kind of go together)
1. “The error is within the estimated precision of 5% based on error in the fall time. The theoretical value for the
viscosity of oleic acid is 2.95 cP. Lab group Taylor Swift and Justin Timberlake’s viscocity was 2.63 cP with an
error of 14%. The large error in these results indicates there is a flaw with the method. One source of error in
this experiment was coordinating the stopwatch start time with when the test tube was turned over. It was
challenging to coordinate these two steps at the same time, leading to random errors.” (assuming you did not
do something wrong here. Maybe everyone else did fine and you know you made a mistake)
2. “The low error indicates that the method used works well for this analysis. The percent precision is
approximately 2% for the colorimeters used due to the absorbance of the different cuvettes. Lab group Michael
Scott and Dwight Schrute found the iron content to be 35 mg, adding additional validity to the result. The high
R2 of the calibration curve and the small percent error indicated the experimental procedure was valid and
Application and Improvement
1. “An improvement to this lab would be to use a longer test tube. The increase in travel time of the marble would
decrease the error introduced by not being able to coordinate the stopwatch/ test tube turn-over event. An
application of this method could be the determination of relative viscosities of household substances such as
honey, corn syrup, and vegetable oil. Or even a comparison of various cooking oils.”
2. “To improve this lab, more accurate measurement of absorbance values could be made with a
spectrophotometer. An alternate application of this experiment is to determine the iron content of juices. Care
should be taken to ensure the fruit juices do not absorb light at the wavelength used to analyze iron.”
An Example Experiment
Steps performed in lab
A student performs an experiment to determine how many M&M candies are in a closed package without opening it.
The student weighs 5 individual candies, weighs the package, and uses the two pieces of information to estimate how
many candies are in the package. Data is shown below.
Mass of M&M packet: 50.293 g
Estimated number of M&M’s: 50.293 g/0.8720 g = 57.68
Actual number of M&M’s: 57
% error: |57.68-57|/57*100% = 1.2%
% precision of mass: 0.005/0.872*100% = 0.5%
The Postlab discussion could look like this:
The number of M&M candies in an unopened snack-sized package was determined using the mass of individual M&M
candies and the mass of the package. Five individual M&Ms were weighed independently, and then an unopened
package of M&M’s was weighed. The average mass of an individual M&M was found to be 0.8720 grams. An unopened
pack was estimated to have 57.68 M&M candies while the actual number in the package was 57, resulting in an error of
1.2%. The percent precision in the mass is estimated to be 0.5% which is more than two times smaller than the percent
error, indicating a possible flaw in the method. The most likely reason that the estimate is high is due to the mass of the
package not being accounted for. Lab mates Bruce Wayne and Peter Parker also overestimated getting 59.82 M&M’s in
a package containing 59, giving a 1.4% error. Their result supports the reasoning that the mass of the package should be
accounted for in the method. To improve the estimation, several packages would be emptied, and an average packaging
mass could be obtained and subtracted from the package mass. This method could also be applied to determining the
number of skittles in snack or fun sized packages.
Some comments on the postlab example above
We could check the label to see how the M&M’s should be in a package. That would be the literature value that
we could compare to. However in this case it is better to actually open the package and check the number of
M&M’s present. This way a theoretical value can be obtained.
It is not important to include the masses of the packages of M&M’s, since that level of detail is not necessary.
I highlighted the sentence above “The average mass of an individual M&M was found to be 0.8720 grams”. This
is actually not a necessary bit of information because the reader doesn’t really need to know. It could be used
however to discuss the percent precision of the measurement, so you could potentially use it.
Postlab grading Rubric
2. Procedural outline.
3. Restate the
whether the results
of the experiment
values help with this)
If not, list possible
sources of error.
4. Theoretical values
and to the work of
5. Improvement and
Grammar, style, etc.
and given in own words
and well written.
Specific to experiment.
Procedure written in
3rd person passive
voice, past tense. 2-3
but not unnecessary
steps or details. 2 pts
Relevant results are
given and discussed.
Key observations count
reactions. Discussion is
concise and orderly. 3
presented that draws
conclusions from data.
Discussion reveals if
objective was met. Any
and sources of errors
are discussed. This will
include percent error
and percent precision or
standard deviation if
possible) to the work of
others, including lab
mates and literature
values were possible. 2
Both a sensible
given. 2 pts
Few spelling and
grammar errors. Text is
well written. No
personal pronouns. 2
Objective does not completely
match the intended purpose of
the experiment. Now well
written. Not specific enough. 1
Procedure written in wrong
voice, lacks important details,
or is too long. 1 pt
Relevant results not presented.
Or not orderly or contains
clearly non-relevant discussion.
Data is presented but
conclusions are not drawn in
Procedure present but
contains multiple errors. 0
Does not match the
intended purpose of the
written. 0 pt
Results and discussion
missing/incomplete. 0 pts
Conclusions are given that are
not supported by data.
Discussion leaves reader
unsure if objective was met or
is not clearly connected to
If errors are present, they are
No conclusions. No
discussion. No connection
to objective. No
consideration of validity of
Comparison not quantitative.
Or work of others presented
but not compared to your
results. Or flawed comparison.
The work of others not
presented or discussed. 0
Only one given or if one is
severely flawed. 1 pt
No sensible improvement
or other application given.
Contains some grammar errors
or personal pronouns. Text is
not well written. 1 pt
Contains many grammar
errors. Text is poorly
written. 0 pts
Things that should not be in a postlab discussion
Unnecessary materials, or a list of materials used (that goes in your lab notebook)
Safety cautions (those go in your lab notebook)
Answers to procedural questions (those go in your lab notebook) unless they are directly related to the required
content of a postlab discussion.
Sample calculations (those go in your lab notebook)
Explanation of calculations: Those are apparent from your sample calculations in your lab notebook
Explanation of any commands or setup that you do on a computer (like pressing a particular button, etc.)
First and second person personal pronouns, possessive adjectives, possessive pronouns: I, me, you, your, yours,
we, us, our, my, mine, … etc.
Example phrases to avoid:
o “The data was recorded.”
— that is obvious right? But you should say when something special
collects the data for you. For instance, “The absorbance values were recorded by a Vernier colorimeter.”
“Next, a Graduated Cylinder was used.” — just say how it was used. Also most of the time, the
glassware that was used does not matter. You would only say this if it was important a graduated
cylinder was used.
“we then determined our percent error ” – just give percent errors, etc. Also wrong voice
” we are going to …”
“In the 1st week’s lab experiment” — Don’t label lab with the number. That doesn’t make sense to
anyone outside of the lab.
“In today’s lab”, “In this week’s lab”…
“we did an experiment”
“our goal was to learn about the different tools…” — Your goal should be to measure/determine
something. You are in the class to learn, yes, but the purpose of the experiment is to do something.
“The average of difference for the mass for part A” – don’t label with labels as in the procedure. This
will not make sense to anyone else.
“Weighed the beaker empty at 30.816 g and reweighed the beaker at 47.608g,” — unnecessary
information for procedure in a postlab
“we set up the burette” – unnecessary, because if you used a burette, you obviously had to set one up.
“A beaker was obtained and cleaned” — you see how that is not necessary as well? And you should
always use clean beakers anyway.
“The two beakers were labeled as “A” and “B”” – What does it matter what you labeled them? The only
time a label will be important is when it is an unknown substance. In which case you have to give how
it was labeled.
“when compared to the chart provided by the instructor” — what chart was that? It will not make sense
to anyone else.
— avoid personal pronouns. Also wrong tense
— when was today? When was this week?
— You don’t say???
Practical Postlab example: Baking a cake
Example 1: not really enough info (also not the language you should use).
I baked a cake, with a recipe I got off of the internet. I baked it in my oven at 375 degrees. I then decorated with
icing. I accidentally dropped the cake on the floor, but I picked it up within 5 seconds, so everything should be fine. I
took the cake to work and most people said it was good. I had fun baking the cake and will do it again soon.
Example 2: Too much information that is not really useful in a summary
On October 12, 2020, an experiment was performed with the objective of determining if 20 people would find
Alton Brown’s Chocolate Devil’s Food Cake better tasting than Betty Crocker’s Super Moist Chocolate cake mix. To
prepare Alton Brown’s Chocolate Devil’s Food Cake, a 13×9 metal pan was preheated in an oven to 325 °F. Then four
ounces of Dutch-process cocoa was whisked into one cup of boiling water and allowed to cool. Separately 10.5 ounces
of dark brown sugar, 5.5 ounces of all-purpose flour, 4 ounces of cake flour, 0.5 teaspoon of Kosher salt, and 1 teaspoon
of baking soda were combined in a stand mixer with paddle attachment. Then 1 cup of vegetable oil …. (continue on
with instructions) … after beating the batter was poured into the preheated pan and baked at 325 °F for 33.4 minutes,
until the cake sprung back when pressed and the internal temperature was greater than 205 °F. The cake was cooled on
a rack for 30.0 minutes and then removed from the pan. After cooling for one hour, the frosting was applied. To prepare
Betty Crocker’s Super Moist Chocolate cake, (continue on with instructions)… . The two cakes were placed in separate
cake containers and placed in a refrigerator at 37 °F. at 8:00 PM. At 9:00 AM on the following day, 20 employees at the
Walmarts on 2315 Madison St, Clarksville, TN 37043 were given 100.0 g samples of both cakes in a blind taste test and
rated the cake on a scale of 1 to 10. The participants were given 20.0 mL of Fresca between the two different samples to
cleanse the palate. The following data was obtained:
Rating Alton Brown’s
… Things continue being excessive and giving way too much information.
Rating Betty Crocker’s Mix
Example 3: A balance of brevity and information. I will put in all the things a postlab should have.
A blind taste test was performed to compare Alton Brown’s Chocolate Devil’s Food Cake1 and Betty Crocker’s
Super Moist Chocolate cake mix.2 The two cakes were prepared as given in their instructions and placed in the
refrigerator. The next day, samples were given in a blind taste test to 20 employees of Walmarts, who were asked to
rate each cake on a scale of 1 to 10. On average Alton Brown’s cake received a rating of 7.75 with a standard deviation
of 1.58, while Betty Crocker’s cake mix received a rating of 6.3 with a standard deviation of 1.65. Clearly there is a
preference for Alton Brown’s cake, with a difference of 1.45, though the difference is within one standard deviation and
the small sample size prevents a definitive conclusion. The preference for Alton Brown’s cake is reasonable, since there
were more fresh ingredients like sour-cream and mayonnaise, which could add to the complexity of the flavor. This
result agrees with a Food network taste test which found a preference for Alton Brown’s Chocolate Devil’s Food Cake
over a generic brand.3 Coworker Timo Boll conducted a similar taste test one week prior and found the Alton Brown’s
cake to be rated 0.6 higher on average than Betty Crocker’s cake mix. This agrees with the current study on the
preference for Alton Brown’s cake, though the differences in the averages still show the results suffer from a small
sample size. To improve the result of the study, a larger sample size is needed, and maybe customers at the Walmarts
can be included. This blind taste test can also be used determine if handmade pizza dough is preferred over prebaked
Sources (normally you would not have 3 references in a postlab. Usually only 1, maybe 2)
3) Some reference
Note: In this example is say “The two cakes were prepared as given in their instructions”. You can not say for example
“the experiment was preformed following the instructions”. You need to summarize the procedure with important
information down to 3 sentences.
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