Passage I (Questions 1-8)

Rationale. An investigator sets out to determine the absorption spectrum of the Co(NH 3)6+3 complex, and to study the effect on the spectrum of substituting different "ligands"--groups surrounding the cobalt--for the original ammonia ligand.

The investigator hopes to gain information about the molecular orbitals in the cobalt complexes. The first step in such a process is to measure the wavelength of maximum absorption for each complex, and to interpret this wavelength in terms of a transition energy.

Procedure. The experimental setup is shown in the next column. The experimenter uses the same cell for each sample, so that the path length will not vary, and each solution is made up to be 0.01 M. By measuring the light transmitted by the sample, a photodetector at the output end of the spectrometer determines the absorbance at each wavelength, and sends this information to the chart recorder. The latter displays absorbance as a function of wavelength; the investigator calculates the wavelength of maximum absorption of each compound.

Analysis of Data. Table 1 shows ranges of wavelength and related energy.

Table 3 shows several possible series of molecular energy levels that might be used to explain the transitions in Table 1 and 2. When a photon matching the difference between two of the levels is absorbed, an electron can be promoted to a higher level.