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Experiment3 - CHEM 241L Spectrophotometric Determination of...

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CHEM 241L: Spectrophotometric Determination of the pKa of the Acid-Base Indicator Bromothymol Blue 69 3 Experiment 3 Spectrophotometric Determination of the pKa of the Acid-Base Indicator Bromothymol Blue Read Sections 9-2 & 9-3 (Chapter 9), Section 11-6 (Chapter 11), and Sections 18-2 & 18-3 (Chapter 18) in Harris, 7 th ed. Take the time to look over the Excel Graphing Tutorial presented in Experiment 0 of your lab manual. Note: You need to bring your laptop to lab for this experiment BACKGROUND & THEORY Beer’s Law & Visible Absorption Spectra The colors that we see are a manifestation of the absorption of light by molecules and materials. In chemistry we often use the absorption properties of molecules as a method of characterizing them. Absorbance ( A ) is defined by the following equation, = I I Log A O (1) In Equation 1, I 0 is the irradiance of the light measured in the absence of an absorbing solution, which in our case is just water, and I is the irradiance of the light measured after it passes through a solution of interest. Absorbance measurements are made using a spectrophotometer, such as the one illustrated in Figure 5. Absorbance measurements can be made at a single wavelength ( λ ) or they can be made at many wavelengths. A plot of absorbance versus wavelength is called a spectrum ( spectra , plural). A spectrum tells us what wavelengths or "colors" the sample absorbs, or by difference, what wavelengths or colors the sample transmits. The percent transmittance (% T) is simply defined as: Reading Assignment
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