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andrade3 - EXPERIMENT 3: ACIDIC MOLECULES (FORMAL)...

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Unformatted text preview: EXPERIMENT 3: ACIDIC MOLECULES (FORMAL) Introduction: ___10__ / 10 pts Procedure: ___4____ / 5 pts Data and Results ___33___ / 42 pts • Table of Maximum Wavelength (λ max ) Data (3 points) • Table of Absorbance-Wavelength Data from Bromothymol Blue spectra at pH 5.00 and pH 10.00 (4 points) • Table of Absorbance-pH Data (3 points) • Table of Absorbance Data for Extract Samples (3 points) • Table of Calculated Concentrations for 8 Buffer Solutions (3 points) • Table of experimental and literature pKa values for bromothymol blue. (4 points) • Absorbance vs. Wavelength plot with the bromothymol blue spectrum obtained at pH 5.00 overlaid with the bromothymol blue spectrum obtained at pH 10.00. (6 points) • Absorbance vs. pH plot (6 points) • First derivative plot and corresponding spreadsheet used to construct the first derivative plot. (6 points) • Concentrations for 8 Buffer Solutions – Accuracy of Values (4 points) Sample Calculations __6____ / 10 pts • Molar absorptivity (5 points) • Concentration of basic form of dye in sample. (5 points) Discussion __9____ / 10 pts • A short discussion of the pKa of bromothymol blue that you experimentally determined and a discussion of how this value compares to the literature pKa value. • Error Analysis: Problems & errors in experiment • Observations and conclusions from the experiment. • Is the discussion section cohesive and well written. 5 Questions: ___17___ / 23 pts Question 1: 5 pts Question 2: 3 pts Question 3: 5 pts Question 4: 4 pts Question 5: 6 pts Other Deductions (i.e., Late Lab Report, incorrect formatting, etc…) - ______ Total Score ___79___ / 100 pts Spectrophotometric Determination of the pK a of the Acid-Base Indicator Bromothymol Blue Francisco Andrade Jessica Booe 6/1/10 Wesley Storm CHEM 241L 402, Morehead Labs Room 301 Pledge Introduction: The objective of this experiment was to first use spectrophotometry to determine the maximum absorbance wavelength for the acidic and basic forms of bromothymol blue, and then to use the maximum absorbance wavelength of the basic form to measure the absorbance of eight different bromothymol blue buffer solutions. This data will be used to construct a graph of absorbance versus pH from which the K a of bromothymol blue will be determined. Bromothymol blue is an acid-base indicator such that the acid and basic forms of the dye are different colors 1 . The acidic form is yellow, while the basic form is blue. As the pH of the solution changes, color changes can easily be observed. The change in color is due to bonds in bromothymol blue reorganizing as the compound accepts or donates protons. Due to dissociation by water, a hydrogen ion is lost, converting bromothymol blue from the acidic state to the basic state. Through the process of electron delocalization, a shift in electrons and bonds allows for the anion to become more stable. When the oxygen regains a proton, the acidic form is seen again. The different colors that are seen are due to the absorption of light by the...
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This document was uploaded on 11/04/2011 for the course CHEM 241 at UNC.

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andrade3 - EXPERIMENT 3: ACIDIC MOLECULES (FORMAL)...

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