Ch.1 The Analyst’S Toolbox Test Bank Answers

Chapter 1

Problem 1.1: Absorption of UV-vis photons usually is associated with electronic transitions—that is, such absorptions cause electrons to move from one energy level to another. In your course on general chemistry, you learned that energy levels are quantized. Based only on that information, what types of peaks (broad or narrow) would you expect to see in a UV-vis spectrum? Explain your answer. Propose some ideas for why we usually observe smooth, broad peaks in UV-vis spectroscopy?

Problem 1.2: Beer’s law states that absorbance = εbc. In this equation, ε is the molar absorptivity (in units of cm^-1∙M^-1), b is the absorbent path length in cm, and c is the concentration in molarity. Estimate ε for the DPK peak seen at 272 nm in Figure 1.2, assuming a standard cell path length of 1.0 cm.

Problem 1.3: Using the molar absorptivity calculated in Problem 1.2, what would be the concentration of DPK required to produce an absorbance of 0.433 at 272 nm?

Problem 1.4: A 13.50 mg sample of impure DPK was dissolved in 50.00 mL acetonitrile and placed in a sample cell with a path length of 0.05 cm. The absorbance was found to be 0.513 at a wavelength of 272 nm. What was the percent purity of the DPK sample? Hint: Use the molar absorptivity calculated in Problem 1.2.

Problem 1.5: What is the underlying basis for the relationship between the absorptivity (strength or height) of the peak and quantitative sensitivity?

Problem 1.6: Estimate ε for the peak seen at 1690 cm^-1 in Figure 1.4. (See Problem 1.2.)

Problem 1.7: This is an open-ended question, so you will need to defend your answer. You are setting up a new analytical laboratory. Which do you purchase first: a UV-vis or an FTIR instrument? In every such instance, a chemist must take into account cost and benefit and also must consider what is needed in his or her analyses. Be sure to explain your answer.

Problem 1.8: In a standard ¹H NMR spectrum, what type of splitting pattern would you expect for each unique proton in 1-chloropropane? 2-chloropropane?

Problem 1.9: The spectra of 2-butanone, methylethylether, and 1-butyne will each exhibit a triplet, a quartet, and a singlet similar to the spectrum seen for diethylmalonate (Figure 1.9). For each of those compounds, explain how you would distinguish it from diethylmalonate using the NMR spectra. Hint: Consider the shielding effects of nearby nuclei.

Problem 1.10: Consider the spectrum shown in Figure 1.11. Looking at the structure of the compound given in the figure, what gives rise to the specific peaks we see? That is, why do you think we observe certain fragments but not others?

Problem 1.11: Both spectra shown in Figure 1.12 exhibit a notable peak at m/z = 30 amu. Postulate at least one fragment that could give rise to that peak in both spectra.

Problem 1.12: Consider what you know about MS and HPLC. Why is it only relatively recently that MS has been used as a detector for HPLC?

Problem 1.13: Consider the chromatogram given in Figure 1.13, noting that peak 7, methamphetamine, and peak 8, 4-methylenedioxymethamphetamine, are not fully separated. Why do you think the experimenters had difficulty in that separation?

Exercise 1.1: Given the relationship absorbance = εbc, where ε is the molar absorptivity (in units of cm^‑1∙M^-1) and c is the concentration in molarity, estimate ε for the peak seen at 356 nm in Figure 1.2.

Exercise 1.2: The molar absorptivities for all three peaks exhibited by DPK in Figure 1.2 have been determined in this chapter. What conjectures can you make about the transitions that give rise to those peaks?

Exercise 1.3: Consider the peak at about 1000 cm^-1 in Figure 1.4.

(a) Convert the wavenumber (cm^-1) to wavelength in micrometers.

(b) Estimate the absorbance of that peak and convert it to percent transmittance.

Exercise 1.4: The Fourier transform is mentioned with respect to two instruments in this chapter. Find a reliable website that describes the Fourier transform and summarize your findings in 250 words or less.

Exercise 1.5: Rationalize the positions of the base peaks of the two spectra shown in Figure 1.12.

Exercise 1.6: What region of the EMS (in Hertz) is utilized by each of the following spectroscopies?