Directions for Experimental Report

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San Diego State University *

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CHEM 200

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Chemistry

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Jan 9, 2024

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CHEM 201 Directions for Experimental Report A formal, typewritten report for each laboratory experiment, not including the unknowns or Le Chatelier’s Principle, is required. These will be due at 11:59 pm the day before your next laboratory session. Late lab reports will receive a zero. A sample lab report is given following these direction. Your formal report is to be divided into sections. Each section should be labeled with its own section heading and should appear in your report in the same general format as shown below: Procedure Use the Procedure section to refer to your Pre-lab pages in your notebook letting the reader know what procedure was followed. Also, list any changes to the procedure from that in the Pre-lab. For example, “See the pre-lab report on page 22 in my laboratory notebook for an outline of the experiment’s procedure. There were no changes made to the experiment.” If you refer to the SDSU lab manual you will receive 0 points for this section Experimental Data The section for experimental data comes next. In some cases it will be appropriate to present your data in a single table or several tables, in other cases the data should be presented in graphs. The appropriate choice will be indicated in the experiment description. Units for numbers should be indicated either in column headings of a table or the axis labels of the graph. If you prefer, tables may be handwritten instead of typed, they must be in pen and neatly done. Graphs should be produced using a computer program such as Excel. This program is generally available on the campus computers and it is to your advantage to learn how to use it. Ask your TA if you need help with Excel; they are prepared to show you how to use it. Calculated Results This section should follow the Experimental Data section. Calculated results should be summarized neatly in tabular form constructed through Excel. No hand-written tables. Use the correct number of significant figures and the proper units in the result. Sample Calculations This section always follows the Calculated Results section. All calculations for an experiment should be done in your lab notebook. Make sure the sample calculations are clearly labeled . Include formulas that were used and units for each type of calculation performed. You will be using Excel to assist you in your calculations but it is recommended to should one typed sample calculation. Discussion & Conclusion The last section is the Discussion & Conclusion section. This section should begin with a sentence that concisely summarizes your results. For example, let’s say the 1

CHEM 201 experiment involved measuring the acid dissociation constant of acetic acid. A concise summary of your results would be something like In this experiment, I determined that the acid dissociation constant, Ka, of acetic acid is 1.510 -3 . This should be followed by a discussion of the quality of this result. First, are the results reasonable based on your understanding of the chemistry involved? Why or why not? Continuing the previous example, you might want to say Acetic acid is a weak acid and 1.510 -3 is a reasonable Ka value for a weak acid. In many cases you will have made multiple measurements and be reporting the average result. If the different measurements give similar values this should give you greater confidence in the accuracy of your result, but if they vary considerably this would be a cause of concern and you should mention this in your discussion. Furthermore, three separate measurements gave Ka values that were quite close, 0.510 -3 , 1.610 -3 , and 2.410 -3 , suggesting that my result is accurate. In some cases, you may be able to compare your result to a literature value. In the acetic acid example you can look up the K a of acetic acid in your text, so the next sentence could be However, the text lists the Ka of acetic acid as 1.810 -5 , which is almost 100 times smaller than the value I determined. This indicates that despite the consistency of our values, the result is not very accurate. Cite the source of your literature value in the following manner: Basic Format for books with Authors: Author, A. A.; Author, B. B. Book Title (italics), Edition (if any); Publisher: Place of Publication, Year; Pagination. Silberberg, M.S.: Amateis, P. Chemistry, The Molecular Mature Of Matter and Change, 7 th Edition, McGraw Hill Education: New York, 2015, p. A-8. Next, you should discuss possible sources of error in the experiment and how this may have affected your results. Of course, if your results are way off, as in the acetic acid example, the first thing you should do is re-check your calculations! If you are still way off, you will want to pay particular attention to the error discussion in your report. However, even if your results are quite good, you still need to include an error discussion because there will always be some error. To think about the error, what you want to do is to go over exactly what you did in the experiment. Each time you made a measurement, there will be an error associated with that measurement. For example, suppose you have to measure out a certain volume of liquid – even if you were very careful, how accurately could you have done that with the equipment you used? +/- 5 mL, +/- 1 mL, +/- 0.2 mL? All the experiments involve reading a number off a scientific instrument. Even if the instrument is working perfectly, the number will be +/- 1 in the 2

CHEM 201 last digit. The instrument also will need to be calibrated correctly. What if this was not done properly, either because of an instrument problem or operator error or the calibration solution was the wrong concentration, how will this affect your result? In addition to errors associated with making measurements, errors will also result from contamination due to dirty glassware, improper handling of stock solutions and so on. An OK discussion of error will list the possible sources of error for the particular experiment being done. A good discussion of error will indicate how the error affects the measurement. For example, an OK error discussion for the acetic acid example might be The fact that my Ka value for acetic acid is so much larger than the text value indicates that there were large sources of error in this experiment. Possible sources of error could be the stock solution of acetic acid (it could have been the wrong acid or the wrong concentration), the dilution of the stock solution (we could have measured out the wrong volumes with the graduate cylinders) or the pH meter (the pH electrode could have not been working properly or we could have calibrated it incorrectly). A good error discussion might be The fact that my Ka value for acetic acid is so much larger than the text value indicates that there were large sources of error in this experiment. The reason we got a Ka value that was too large could be because the solution we measured the pH of was actually much more concentrated that we thought it was. This would have occurred if the stock solution was more concentrated than it was supposed to be or we added a smaller amount of water than we were supposed to when doing the dilution. Another reason the Ka could be too high was that the pH meter gave a pH value that was smaller than it should be. This would occur of the pH electrode wasn’t working properly. We had great difficulty in calibrating the meter and even after getting help from our TA the meter indicated that the electrode was ‘bad’. Given this, I believe our pH values are likely the greatest source of error in this experiment. The last part is concluding your experimental results with the purpose of the experiment. The conclusion is a final summary of what was learned in the experiment based on the theory and the purpose. 3

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CHEM 201 Sample Lab Report Iwanna A. Realbad Lab Partner: Bea S. Okwithme March 02, 2022 Homogeneous Equilibrium Experiment Procedures: See the pre-lab report on page 25 of my laboratory notebook for an outline of the general procedure. In the actual experiment, no changes to the written procedure were made. Experimental Data: Copied from Excel Sheet Calculated Results: Copied from Excel Sheet Tube mL SCN mL Fe(NO3)3 mL Fe(III)+mL KSCN % T 1 10 0 10 95.8 2 8 2 10 49.2 3 6 4 10 38.7 4 5 5 10 39.8 5 4 6 10 39.7 6 2 8 10 55.4 7 0 10 10 96.3 4 Tube mole fracDon ABS 1 0 0.0186 2 0.2 0.308 3 0.4 0.412 4 0.5 0.400 5 0.6 0.401 6 0.8 0.256 7 1 0.0160

CHEM 201 Copied from Excel Sheet Sample Calculations Screenshot from Excel Sheet Screenshot from Excel Sheet Trial Two Sample Calculations: • Mole Fraction: = 0.2 • Absorbance: • [Fe(SCN) 2+ ] : = 6.55 x 10 -5 Total Free Kf Kf average Tube [Fe(SCN) 2 ] CSCN CFe Fe(III) SCN- 109.91 1 4.0E-06 0.002 0 0 0.0020 N/A 2 6.55E-05 0.0016 0.0004 0.0003 0.0015 127.70 3 8.77E-05 0.0012 0.0008 0.0007 0.0011 110.72 4 8.51E-05 0.001 0.001 0.0009 0.0009 101.71 5 8.54E-05 0.0008 0.0012 0.0011 0.0007 107.17 6 5.46E-05 0.0004 0.0016 0.0015 0.0003 102.23 7 3.48E-06 0 0.002 0.0020 0 N/A 2 mL 10 mL 2 − log (49.2) = 0.308034897 0.308034897 4700 5

CHEM 201 • c SCN : = 0.0016 • c Fe : = 0.0004 • Fe(III): 0.0004 — 6.55 x 10 -5 = 0.0003 • SCN — : 0.0016 — 6.55 x 10 -5 = 0.0015 • K f : _________6.55 x 10 -5. = 127.70 (0.0015 x 0.0003) • K f average: 127.70 + 110.72 + 101.71 + 107.17 + 102.23 = 109.91 5 Discussion: This experiment yielded the formation constant. The equation used to determine the formation constant: . The average k f for determined was 109.91. The mole fraction where the maximum absorbance occurs is 0.40 of Fe 3+ . The range in which absorbance is at its highest is when the mole fraction is between 0.40 and 0.60. The specific concentrations used in this experiment show the exchange between the equilibrium system when the volumes of the ions are changed. According to the Le Chatelier’s principle when a change is applied to a system in equilibrium, it will shift to counteract the effect. An increase in concentration of the reactants causes a shift to the right which would decrease the formation constant value. As the mole fraction of Iron(III) increased the absorbance values decreased. Conclusion In this experiment, complex ions were evaluated. The reaction observed was the reaction of Iron(III) with thiocyanate, which transforms to a red-colored solution. The absorbance of the reactions using different levels of concentration were gathered using spectrophotometry. Furthermore, the mole fraction of Iron(III) was determined using a Excel graph to displayed the absorbance as a function of the mole fraction of Iron(III). The objective was to find the formation constant, which describes the formation of the complex ion from its central ion and attached ligands. The larger the value means the more stable or equal the reaction will be. The formation constant was calculated through the concentration of reactants and products. An increase in concentration of the reactants causes a shift to the right which would decrease the k f value. As the mole 0.002 x 8 mL 10 mL 0.002 x 2 mL 10 mL 6

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CHEM 201 fraction of Iron(III) increased the absorbance values decreased. This was observed on the graph and through the collection of data. It was determined that the maximum absorbance occurred when the mole fraction of Iron(III) was 0.40. 7