UV Visible Spectroscopy Questions and Answers

Answer: A

The pairing is A-P, B-Q, C-R, and it is simply the four transitions in order of the energy each requires. Because only the last two fall inside the range an ordinary instrument covers, they are the only ones an analyst normally sees - which is why saturated compounds make such good solvents and why the technique is so closely associated with conjugation and lone pairs.

Answer: B

The pairing is A-P, B-Q, C-R. Two terms describe movement along the wavelength axis and two along the intensity axis, and a real substituent or solvent change usually produces some of each. Having a vocabulary for both axes is what lets a spectral change be described precisely enough for someone else to reproduce the observation.

Answer: C

The first three are correct and the fourth is not, so three match. Cyclohexane is a saturated hydrocarbon with no accessible transitions and is transparent to about 200 nanometres, which is why it is a standard ultraviolet solvent. A cut-off follows directly from the chromophores a solvent contains, so a molecule with no double bonds and no conjugated lone pairs will always be transparent furthest.

Answer: D

A strong short-wavelength band is the allowed pi to pi star transition of the conjugated system, and a weak long-wavelength one is the forbidden n to pi star of a carbonyl's lone pair. A saturated compound would show nothing, a bare aromatic ring gives a characteristic weak fine-structured band near 255, and concentration affects height rather than position. Reading intensity and position together identifies the transitions, and the transitions identify the groups present.

Answer: A

A shift to shorter wavelength in the more polar solvent is the signature of an n to pi star band, whose ground-state lone pair is stabilised by hydrogen bonding. A pi to pi star band would move the other way. The sigma transitions lie far outside the measurable range. This single experiment distinguishes the two transitions that both fall in the accessible region, which is why a change of solvent is often the quickest structural question to ask. Working one numerical case through both cases side by side fixes the distinction better than any rule of thumb.

Answer: B

Curvature appearing only at high absorbance is the classic signature of stray light, which sets a ceiling on the measurable absorbance, though molecular interaction at high concentration produces the same shape. Wavelength drift, cell mismatch and a wrong blank would affect the whole line rather than only its upper end. Where on a calibration a departure appears is what identifies its cause, and the top end points at these two mechanisms alone. Checking the units of each quantity catches a wrong formula before the arithmetic begins.

Answer: C

Absorbances add, so measurements at two wavelengths give two equations in the two unknown concentrations, provided the absorptivity ratios differ between them. Dilution reduces both equally, a narrower slit cannot separate overlapping broad bands, and an isosbestic point gives a total rather than the individual values. Counting unknowns against independent measurements settles how much data any determination needs. A single measurement chosen to respond to the one asymmetry beats several that respond to shared behaviour.

Answer: D

Choosing wavelengths where one compound dominates at each makes the two equations very different, so a small measurement error produces a small error in the answer. Steep sides make readings wavelength-sensitive, equal absorbances would make the equations degenerate, and being beyond the cut-off would make measurement impossible. The quality of a simultaneous determination depends on how independent the equations are, and that is what the wavelength choice controls.

Answer: A

Cofactors such as the reduced nicotinamide dinucleotide absorb strongly at 340 nanometres while their oxidised forms do not, so the reaction's progress appears directly as a changing absorbance. Proteins absorb weakly near 280 rather than in the visible, and neither heat nor refractive index is being measured. Following a reaction through a species whose absorption changes is the general pattern, and it needs no sampling, no quenching and no separation. A single measurement chosen to respond to the one asymmetry beats several that respond to shared behaviour.

Answer: B

Tryptophan and tyrosine, with their aromatic rings, are the principal absorbers near 280 nanometres, which is why the band's size depends on a protein's composition and why an absorptivity must be known or estimated for each one. The backbone absorbs far shorter, around 190 to 220, and metals and disulphides contribute little. That a routine concentration measurement depends on the particular amino acids present is a limitation worth knowing before the number is trusted.