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Chem 265 [1] Assignment Four with Answers 1) If an analyte that is to be measured by UV-vis absorption spectroscopy has a molar absorptivity of 5.6 x 105L/mol.cm at the wavelength that is chosen for its measurement. If the smallest absorbance that can be measured is 0.002 and the sample cuvette has a pathlength of 1.00 cm, what is the lowest concentration of the analyte that can be measured? π΄ [3] 0.002 = 5.6 π₯105 πΏ/πππ.ππ∗1 ππ = 4 π₯ 10−9 M πΈπ΅ ππππ = 2) A geochemist wishes to estimate the volume of an irregularly shaped pool of water. This task is accomplished by placing a known amount of highly coloured dye into this solution, letting it mix throughout and then measuring the absorbance of the dye in a sample of the pool water. For example, a geochemist used 1.00 L of a dye solution that had an absorbance of 0.768 when it was measured in a 1.00 mm cuvette at 450 nm. They put all of the dye solution in the pool and allowed it to be mixed around well. They then took a sample of the pool water, which was then a very dilute solution of the dye, and measured the absorbance in a 10.00 cm cuvette. The absorbance was 0.142 at 450 nm. What is the volume of the pool? Initially: A = Ebc = 0.768 = E. 0.100 cm . conc 0.768 E = 0.10 ππ.ππππ A = Eb. Dil So 0.142 = So ππππ π·ππ = 0.768∗10.00 ππ 0.10 ππ∗0.142 0.768 0.10 ππ.ππππ . 10.00 cm . dil = 541 So the volume has changed by a factor of 541, so the volume of the pool must be 541 L. [2] 3) What is the energy of a photon that has a wavelength of 500 nm, if the refractive index of the medium it is travelling through is 1.00? Given that the refractive index is 1, then c/v = 1 so speed = 3.0 x 108 m/s E=hc/λ h = 6.626 x 10-34J.s E= h = 6.626 x 10−34J.s∗3.0 x 108 m/s 500 x 109 m = 3.98 x 10-19 J What is the frequency of this light? ο΅= [3] c ο¬ ο½ 3.00x10 8 m sec -1 ο½ 6.00 x1014 s ο1 -9 500 x 10 m 4) The following results were obtained by a clinical chemist when measuring morphine at 285 nm and using a square cuvette as the sample holder. Morphine Concentration (M) 5.00 x 10-5 1.00 x 10-4 2.0 0 x 10-4 5.0 0 x 10-4 Sample measurement 1 Sample measurement 2 Sample measurement 3 Absorbance 0.229 0.308 0.467 0.942 0.615 0.617 0.613 Prepare a Beers law plot for these data using the standards. Use this plot to determine the concentration of morphine in the sample. Give your result with a 95 % confidence interval, as it would go to a client. 0.00005 0.0001 0.0002 0.0005 0.229 0.308 0.467 0.942 532 539 542 5.131601 0.615 0.617 0.613 AV 0.000294 0.000295 0.000292 0.000293667 1.52753E06 SD CI 3.79489E06 CI (2.94 ± 0.04)x 10-4 M 1 y = 1584.6x + 0.1498 R² = 1 0.9 0.8 0.7 0.6 0.5 Series1 0.4 Linear (Series1) 0.3 0.2 0.1 0 0 [0.5] [0.5] 0.0001 0.0002 0.0003 0.0004 0.0005 0.0006 Is a solution of morphine coloured? No – it is a white powder and gives a colourless solution Would this be analyzed by visble or UV absorption? UV – we used 285 nm here