Fall 02. Exam2 key

Fall 02. Exam2 key - Chemistry 2101 Introductory Analytical...

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Chemistry 2101 Introductory Analytical Chemistry Fall 2002 Sample Hour Exam III Solutions 1 (20 points) (a) What is an isosbestic point? Explain why an isosbestic point is observed. One or more isosbestic points are obtained by the overlap the UV-Vis spectra of solutions containing two absorbing species if the sum of the concentrations of the two species in all the solutions is the same . The isosbestic point occurs at the wavelength(s) at which the two absorbing species have the same molar absorptivities, . (b) 50 mL of an acid base indicator solution is titrated with NaOH. The titration is followed spectrophotometrically. The equivalence point is reached upon addition of 200 mL of a NaOH solution. Overlapping all measured spectra in one figure cannot give an isosbestic point. Why is this so and how could it be corrected? Answer this question as specifically as possible. The addition of the strong base increases the volume of the solution significantly (factor 5 by the time the equivalence point is reached). To correct for this, all the absorbance values in a spectrum obtained upon addition of x mL of NaOH solution must be multiplied by a factor of (x mL + 50 mL)/50 mL. Unless further complications occur, an isosbestic point should be obtained the corrected spectra are overlapped. (c) What other reasons could be responsible for the absence of an isosbestic point in the above titration? Two trivial reasons are the faulty preparation of the solutions and experimental errors of the spectrophotometric measurements. Give two other possible reasons for the absence of an isosbestic point and explain for each why the isosbestic point is not observed. * The solution is too concentrated (Lambert-Beer law is not valid any more).
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Fall 02. Exam2 key - Chemistry 2101 Introductory Analytical...

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