GenChem

1

A lesson designed to guide you A lesson designed to guide you through the process of composing a through the process of composing a

suitable lab reportsuitable lab report

Report Writing Guide

GenChem

OverviewThis 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.

GenChemSection OrderSection 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 WritePreparing to Write

First Question to ask yourself:What were the major goals of today’s lab?

1.Determine the wavelength of maximal absorbance (λmax).

2.Determine the molar absorptivity at λmax (ԑ).

GenChemIntroductionIntroduction

• 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

GenChemIntroduction: OutlineIntroduction: Outline

Question to ask yourself:For each goal, list the major techniques, formulas, and chemicals used:1.λmax

• Absorption Spectroscopy• Food dye

2.ε• Parallel dilution• Absorption Spectroscopy• Excel/trendline• Beer’s law (graph A vs C)• Calipers- path length

GenChemIntroduction: Goal 1Introduction: Goal 1

In a paragraph, introduce Goal 1. Include all items on your list.

– Introduce Goal 1. Define λmax 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 λmax?

GenChemIntroduction: Goal 1Introduction: Goal 1

The first goal was to determine the λmax of the assigned food dye. The λmax 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 λmax was determined using Absorption Spectroscopy, a technique that measures the wavelengths of light absorbed by a colored solution. The λmax was then used to conduct a Beer’s Law analysis in Goal 2. 

GenChemIntroduction: Goal 2Introduction: 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 2Introduction: 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 λmax) 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.

GenChemIntroductionIntroductionThe first goal was to determine the λmax of the assigned food dye.

The λmax 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 λmax was determined using Absorption Spectroscopy, a technique that measures the wavelengths of light absorbed by a colored solution. The λmax 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 λmax) 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: Helpful hintsIntroduction: 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. 52-55 for more help

with scientific writing.

GenChem

ProceduresProcedures

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

report.

GenChemCalculationsCalculations

• 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: OutlineCalculations: Outline

Question to ask yourself:For each goal identify the important calculations that should be included.

1.Determine λmax

• Dilution of stock dye solution

2.Determine ԑ at λmax.• Parallel (or serial) dilution to produce standards.• Calculation of a value for ԑ.

GenChem

CalculationsCalculationsYour presentation of your calculations should always use a three step format:•Write out the equation•Plug in your numbers•Give the answerThis 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 Calculations: Goal 1

For a dilution the product of the concentration times the volume is constant.

•C1V1 = C2V2

•(5.37 x 10-3 M)(1.00 mL) = C2(100.0 mL)

•C2 = 5.37 x 10-5 M

GenChem

Calculations: Goal 2Calculations: 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 2Calculations: 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 HintsCalculations: 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

GenChemResultsResults

• 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. 67-8 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: OutlineResults: Outline

Question to ask yourself:For each goal identify the important results and supporting data that should be included.1.Determine λmax

• Identity of dye• Concentration of dye• Value for λmax

2.Determine ԑ at λmax.• Parallel (or serial) dilution to produce standards.• Calculation of a value for ԑ.

GenChemResults: Goal 1Results: 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 10-5 M

λmax : 610 nm

GenChem

Results: Goal 1Results: Goal 1Graphs are a good way to display data

Figure 1: Visible-UV spectrum of indigo carmine

GenChemResults: Goal 2Results: 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 10-50.943

8:10 4.22 x 10-5 0.782

6:10 3.16 x 10-5 0.577

4:10 2.11 x 10-5 0.392

2:10 1.05 x 10-5 0.181

GenChemResults: Goal 2Results: 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.

ԑ at 610 nm = 5.50 x 105 abs/cm∙M

GenChem

Results: Helpful HintsResults: 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

GenChem

DiscussionDiscussion

• 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.

GenChemDiscussion: GeneralDiscussion: 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 Discussion: Goals

The λmax 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 λmax, 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.

GenChemDiscussion: Data qualityDiscussion: 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.

GenChemDiscussion: Data qualityDiscussion: Data quality

The λmax 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 λmax. 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 0.1-1.0. 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 0.99. And although the y-intercept should be zero, the small value of 0.025 indicates minimal error.

GenChem

DiscussionDiscussion

• See Technique book pp. 68-69 for more help with writing a Discussion.