GenChem 1 A lesson designed to guide you through the process of composing a suitable lab report Report Writing Guide

GenChem Overview This lesson will review the process you need to use to compose a proper report. Each section of the report is covered separately. The experiment covered is the first experiment performed this semester. For each section you will be presented with the questions you need to ask yourself as you compose your report.

GenChem Section Order The order in which the sections of the report should be composed is not the same as the order of the finished report. Order of Sections in Report Introduction Procedures Results Calculation Discussion Recommended Order of Composition Calculations Results Procedures Introduction Discussion

GenChem Preparing to Write First Question to ask yourself: What were the major goals of today’s lab? 1.Determine the wavelength of maximal absorbance (λ max ). 2.Determine the molar absorptivity at λ max ().

GenChemIntroduction From p. 65 of your Technique Book: Introduction. A description of what is already known, the plan of action and expected outcomes. From p. 71 of your Technique Book: Introduction. A description of: the experimental objective(s) pertinent background information expected outcomes, if any

GenChem Introduction: Outline Question to ask yourself: For each goal, list the major techniques, formulas, and chemicals used: 1.λ max Absorption Spectroscopy Food dye 2.ε Parallel dilution Absorption Spectroscopy Excel/trendline Beer’s law (graph A vs C) Calipers- path length

GenChem Introduction: Goal 1 In a paragraph, introduce Goal 1. Include all items on your list. –Introduce Goal 1. Define λ max and give expected outcomes. –Plan of action. How was Goal 1 achieved? Introduce techniques/formulas/chemicals in context. –Relevance of results. What can you do with your newly found λ max ?

GenChem Introduction: Goal 1 The first goal was to determine the λ max of the assigned food dye. The λ max of a substance is the wavelength of light with the highest absorbance and is the wavelength opposite the color seen by the human eye. The λ max was determined using Absorption Spectroscopy, a technique that measures the wavelengths of light absorbed by a colored solution. The λ max was then used to conduct a Beer’s Law analysis in Goal 2.

GenChem Introduction: Goal 2 In a paragraph, introduce Goal 2. Include all items on your list. –Introduce Goal 2. Define ε and give expected outcomes. –Plan of action. How was Goal 2 achieved? Introduce techniques/formulas/chemicals in context. –Relevance of results. What can you do with your newly found ε?

GenChem Introduction: Goal 2 Goal 2 was to find ε for the assigned dye. ε is a measure of how strongly a substance absorbs light at a particular wavelength and is a large value for dark colorful substances. In Beer’s Law, ε is the proportionality constant that relates absorbance to concentration (A=εbC). To determine ε, Absorption Spectroscopy was used to measure absorbance values (at the λ max ) of several dilutions of dye with known concentrations. An Excel graph of A vs. C was generated so slope=εb. The slope was given by the Excel trend line and the path length (b) of the cuvette was measured with calipers. ε can be used to determine the concentration of food dye in unknown solutions.

GenChemIntroduction The first goal was to determine the λ max of the assigned food dye. The λ max of a substance is the wavelength of light with the highest absorbance and is the wavelength opposite the color seen by the human eye. The λ max was determined using Absorption Spectroscopy, a technique that measures the wavelengths of light absorbed by a colored solution. The λ max was then used to conduct a Beer’s Law analysis in Goal 2. Goal 2 was to find ε for the assigned dye. ε is a measure of how strongly a substance absorbs light at a particular wavelength and is a large value for dark colorful substances. In Beer’s Law, ε is the proportionality constant that relates absorbance to concentration (A=εbC). To determine ε, Absorption Spectroscopy was used to measure absorbance values (at the λ max ) of several dilutions of dye with known concentrations. An Excel graph of A vs. C was generated so slope=εb. The slope was given by the Excel trend line and the path length (b) of the cuvette was measured with calipers. ε can be used to determine the concentration of food dye in unknown solutions.

GenChem Introduction: Helpful hints Only include information that is known before lab. No results, observations, etc. 200 word maximum for one week lab. Paragraph format. Subheadings okay. Give items in context. No lists. See Technique Book pp. 65, 67 and 71 for more help with writing an Introduction See Technique Book pp for more help with scientific writing.

GenChemProcedures Also known as “experimental”. This section will be discussed with the next report.

GenChemCalculations From p. 65 of your Technique Book: Calculations. Examples of all calculations performed to manipulate the numerical data. Include one example for each type. From p. 70 of your Technique Book: One example for each type of calculation. Each example should consist of three parts: –The formula that describes the calculation –The formula in use –The answer

GenChem Calculations: Outline Question to ask yourself: For each goal identify the important calculations that should be included. 1.Determine λ max Dilution of stock dye solution 2.Determine at λ max. Parallel (or serial) dilution to produce standards. Calculation of a value for.

GenChemCalculations Your presentation of your calculations should always use a three step format: Write out the equation Plug in your numbers Give the answer This method ensures that your instructor can follow your thought process during the calculation. Do not forget to add units and watch significant figures during your calculations!

GenChem Calculations: Goal 1 For a dilution the product of the concentration times the volume is constant. C 1 V 1 = C 2 V 2 (5.37 x M)(1.00 mL) = C 2 (100.0 mL) C 2 = 5.37 x M

GenChem Calculations: Goal 2 The first calculation of goal 2 is the same as that of goal 1: dilution. Since we have already shown this calculation, we do not need to show it again. If we calculated the concentration a different way, then we would need to show a second calculation.

GenChem Calculations: Goal 2 The second calculation we have listed is density. We obtained this value from the graph, but it would be helpful to make a note in the calculations so that the reader does not think you forgot one: Density of known plastic: Determined from the slope of the graph

GenChem Calculations: Helpful Hints Include a calculation for all types of calculations that you do, but only include one example for the type of calculation Watch significant figures and units

GenChemResults From p. 65 of your Technique Book: Results. An organized presentation of the observations and calculated results. This section is frequently dominated by figures, tables and graphs. It will contain little to no text. From pp of your Technique Book: Results. An objective and clear description of experimental findings following an appropriate order and including relevant: –Tables –Graphs –Figures

GenChem Results: Outline Question to ask yourself: For each goal identify the important results and supporting data that should be included. 1.Determine λ max Identity of dye Concentration of dye Value for λ max 2.Determine at λ max. Parallel (or serial) dilution to produce standards. Calculation of a value for.

GenChem Results: Goal 1 Sometimes, it makes more sense to list numbers and answers in a clear order rather than putting them in a table. Dye: Indigo carmine Description: a clear, blue solution Concentration: 5.37 x M λ max : 610 nm

GenChem Results: Goal 1 Graphs are a good way to display data Figure 1: Visible-UV spectrum of indigo carmine

GenChem Results: Goal 2 For repetitive and/or related data a labeled table is best. Compile all of the items on your list in a logical order. Table 1. absorbance of standard solutions * Ratio of stock solution volume to final solution volume ** absorbance values measured at 610 nm Dilution*Concentration (M)Absorbance (abs)** none 5.37 x : x : x : x : x

GenChem Results: Goal 2 It is often a good idea to display this information in a graph as well. If we plot the absorbance on the x-axis and the concentration on the y- axis, the slope will be the molar absorptivity of the dye. Graph 1. absorbance vs concentration of standard food dye solutions. at 610 nm = 5.50 x 10 5 abs/cm∙M

GenChem Results: Helpful Hints Only include results that are important to the conclusions that you make Be careful about the amount of significant figures you have, especially when copying tables that you’ve made from Excel

GenChemDiscussion From your Technique Book: Discussion. Analysis of the data and presentation of conclusions regarding original hypothesis. This should include a defense of the conclusions citing the results and a discussion of the reliability of the data used.

GenChem Discussion: General Questions to ask yourself: For each goal –What is the result? (Cite data/give examples). –What does it mean? –Does it make sense? –Explain the chemistry. –Is it what was expected? –Is it consistent with your observations? –Comparisons to literature or peers. –If something went wrong: What were you expecting and why? What went wrong? How do you know? How could you fix it next time?

GenChem Discussion: Goals The λ max of Blue #1 was 610 nm which lies in the orange region of the visible light spectrum. As expected, the orange color absorbed is opposite the blue color observed. Using the λ max, the ε for Blue #1 was found to be 55,000 M-1cm-1. This is a larger ε than the other three dyes, which means Blue #1 is the best at absorbing light. This is consistent with the observation that Blue #1 had the darkest appearance out of the four dyes.

GenChem Discussion: Data quality In a paragraph, discuss the quality and reliability of your data: –Discuss pros/cons/accuracy/precision of tools/techniques. –Use technique book! –Be specific and give examples. –Suggest fixes for the problems you identify and additional experiments to improve findings. –Avoid discussing general human error. Only specific, documented errors should be addressed.

GenChem Discussion: Data quality The λ max value seems reliable because the spectrometer gives readings that vary by a maximum of 1 nm. Also, 610 nm is an exact match to the class average λ max. The ε value also came close to the class average ε of 55,000 M-1cm-1. The calipers used to measure the path length are accurate to 0.01 cm and the volumetric kit used for dilutions measures volumes accurate to 0.01 mL. The Absorbance values collected were all within the spectrometer’s effective range of Parallel dilutions were chosen over serial dilutions to obtain a graph with well distributed points. Parallel dilutions also minimize error propagation in consecutive dilutions. The resulting calibration curve showed a strong linear correlation with an R2 value of And although the y-intercept should be zero, the small value of indicates minimal error.

GenChemDiscussion See Technique book pp for more help with writing a Discussion.