Spectroscopic Techniques Questions and Answers

Answer: A

Correct answer: absorbs ultraviolet radiation

Ordinary glass is opaque below about 350 nm, so it would absorb the very radiation being measured and no light would reach the detector across most of the ultraviolet range. Fused quartz transmits down to about 190 nm and is used instead. Glass is perfectly adequate in the visible region, and plastic cells serve there for routine work.

Answer: B

Correct answer: 2000 cm\( ^{-1} \)

Wavenumber is the reciprocal of the wavelength expressed in centimetres:

The conversion is worth committing to memory as \( \bar{\nu} = 10^4/\lambda_{\mu m} \). Wavenumber is used in infrared spectroscopy because it is directly proportional to energy, so the axis is linear in the quantity that matters.

Answer: C

Correct answer: 1.0

Ten per cent transmittance means \( T = 0.1 \), so

The scale is logarithmic, so each further factor of ten removed from the beam adds one to the absorbance: 1 per cent transmittance gives 2, and 0.1 per cent gives 3. Above about 2 the transmitted light is so small that stray light and detector noise dominate, which is why samples are diluted into the 0.2 to 1.0 range.

Answer: D

An absorbance of two means the incident intensity has been divided by a hundred. Readings much above two are unreliable because so little light reaches the detector that stray light and noise begin to dominate the measurement.

Answer: A

Correct answer: 0.10

Applying the Beer-Lambert law,

The units cancel exactly: litres per mole per centimetre, times moles per litre, times centimetres. A molar absorptivity of 5000 is moderately strong, and it means a solution of only twenty micromolar is measurable in an ordinary cell.

Answer: B

Correct answer: 0.40

Absorbance is proportional to path length, so quadrupling the cell length quadruples it:

Long-path cells are used exactly this way to bring a weakly absorbing sample into the range where the measurement is most precise, and gas cells with multiple reflections achieve path lengths of many metres by the same principle.

Answer: C

Correct answer: 0.20

Absorbance is proportional to concentration, so diluting to one third divides it by three:

This linearity is the whole basis of quantitative absorption spectrometry, and it is why absorbance rather than transmittance is the quantity plotted - transmittance would have gone from 25 to 63 per cent, which is not proportional to anything.

Answer: D

Correct answer: degenerate or infrared inactive

Benzene has \( 3 \times 12 - 6 = 30 \) modes, but its high symmetry reduces the number of distinct bands enormously. Many modes come in degenerate pairs of identical frequency and so give one band between them, and many others produce no change in dipole moment and are infrared inactive altogether - though several of those appear in the Raman spectrum. Symmetry, not detector sensitivity, is the reason.

Answer: A

Correct answer: 4

Carbon dioxide is linear, so the formula is

The four are the symmetric stretch, the asymmetric stretch, and two degenerate bends at right angles to each other. Only three frequencies appear, since the bends coincide, and only two bands are seen in the infrared, since the symmetric stretch produces no dipole change.

Answer: B

Correct answer: 4

The multiplicity is \( n + 1 \), so five lines mean

equivalent neighbouring protons, giving a quintet with intensities 1:4:6:4:1. The middle methylene of a propyl chain flanked by a methyl and another methylene shows exactly this pattern.