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Chapter 15 ULTRAVIOLET-VISIBLE SPECTROSCOPY AND MASS SPECTROMETRY 15.1 According to the Lambert-Beer law, where A is the absorbance of the solution, E is the molar extinction coefficient, is the concentration of the solution, and I is the path length of the cell. A = (1.80 X 10⁵ X 10 -6 M)(1 cm) = 0.349. 15.2 = A/cl = (0.153)/[(0.0014 g)/(182 g M⁻¹)][1 cm] = 1.99 X 10⁴ M⁻¹cm⁻¹ 15.3 C = A/εl C = (0.643)/(27,000 cm) = 2.38 X 10⁻⁵ M 15.4 The small molar absorptivity and the wavelength indicate that the transition is 15.5 The absorption at 213 nm is due to a transition (higher energy, larger ε) and that at 320 nm is due to an transition (lower energy, smaller ε). 15.6 a) The ketone on the left is conjugated and should absorb at longer wavelength than the ketone on the right, which is not conjugated. b) The ketone on the right is conjugated and should absorb at longer wavelength than the aldehyde on the left, which is not conjugated. c) The triene on the left has three conjugated double bonds and should absorb at longer wavelength than the triene on the right, which has only two conjugated double bonds. d) Cyclohexanone has an absorption for a transition in the accessible UV region, whereas the alcohol shows no such absorption. 15.7 The exact mass of butane is 58.0783 and that of acetone is 58.0419. Since the molecular masses of these compounds differ by several hundredths of a mass unit, they can be easily distinguished by a high resolution mass spectrometer. 226