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

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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 quantitatively. 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 not acceptable 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. Acceptable • 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 different” • 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.) Objective statement 1. “A relative viscosity scale was established, then use this scale to determine the relative viscosity of an unknown solution.” 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 Procedural Outline 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” Results 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 executed well.” 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 1. Objective 2 pts 2. Procedural outline. 2 pts 3. Restate the relevant results. 3 pts Clearly explain whether the results of the experiment are reasonable (theoretical/accepted values help with this) If not, list possible sources of error. 4. Theoretical values and to the work of others. 5. Improvement and Application Grammar, style, etc. 2 pts 2 pts 2 pts 2 pts Full credit Objective understood and given in own words and well written. Specific to experiment. 2 pts Procedure written in 3rd person passive voice, past tense. 2-3 sentences containing important information but not unnecessary steps or details. 2 pts Relevant results are given and discussed. Key observations count when describing reactions. Discussion is concise and orderly. 3 pts Logical reasoning presented that draws conclusions from data. Discussion reveals if objective was met. Any discrepancies/errors and sources of errors are discussed. This will include percent error and percent precision or standard deviation if possible. Results compared quantitatively (where possible) to the work of others, including lab mates and literature values were possible. 2 pts Both a sensible improvement and another application given. 2 pts Few spelling and grammar errors. Text is well written. No personal pronouns. 2 pts Some credit Objective does not completely match the intended purpose of the experiment. Now well written. Not specific enough. 1 pt Little/no credit 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 logical/orderly way. 2-1 pts Procedure present but contains multiple errors. 0 pt Does not match the intended purpose of the experiment. Poorly 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 objective. If errors are present, they are not discussed. No conclusions. No discussion. No connection to objective. No consideration of validity of results. Comparison not quantitative. Or work of others presented but not compared to your results. Or flawed comparison. 1 pt The work of others not presented or discussed. 0 pts Only one given or if one is severely flawed. 1 pt No sensible improvement or other application given. 0 pts 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.” o "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. o "we then determined our percent error " – just give percent errors, etc. Also wrong voice o " we are going to ..." o "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. o "In today’s lab", “In this week’s lab”… o "we did an experiment" o "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. o "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. o "Weighed the beaker empty at 30.816 g and reweighed the beaker at 47.608g," -- unnecessary information for procedure in a postlab o "we set up the burette" – unnecessary, because if you used a burette, you obviously had to set one up. o “A beaker was obtained and cleaned” -- you see how that is not necessary as well? And you should always use clean beakers anyway. o "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. o "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 13x9 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 ...