Question 1

Which of the following statements best explains why 1,3,5-hexatriene absorbs light at a longer wavelength than 1,3-butadiene in UV-Vis spectroscopy?

Accepted Answer

1,3,5-hexatriene has a greater degree of conjugation, which decreases the energy gap between the HOMO and LUMO.

Question 2

A chemist measures the absorbance of a solution at 350 nm and finds A = 0.750. If the path length is 1.00 cm and the molar absorptivity (ε) is 1500 L·mol⁻¹·cm⁻¹, what is the concentration of the solution?

Accepted Answer

2.14 × 10⁻⁴ mol·L⁻¹

Question 3

Which region of the electromagnetic spectrum has the highest energy per photon?

Accepted Answer

Gamma rays

Question 4

A student is analyzing two compounds: Compound A (no conjugated double bonds) and Compound B (a conjugated diene). Which statement is most accurate regarding their UV-Vis absorption?

Accepted Answer

Only Compound B will show significant absorption in the UV-Vis region due to its conjugated pi system.

Question 5

According to the Beer-Lambert Law, which of the following changes will NOT affect the absorbance (A) of a solution?

Accepted Answer

Using a solvent that does not absorb at the measured wavelength

Question 6

A solution absorbs maximally at 520 nm. According to the visible light spectrum, what color would this solution most likely appear to the human eye?

Accepted Answer

Orange

Question 7

Which of the following best describes the function of the monochromator in a UV-Vis spectrophotometer?

Accepted Answer

It selects a specific wavelength of light to pass through the sample.

Question 8

A chemist uses the Woodward-Fieser rules to estimate the λmax of a conjugated diene with two alkyl substituents and one exocyclic double bond. Which of the following best describes the process?

Accepted Answer

Add the base value for the diene to the increments for each substituent and exocyclic bond.

Question 9

Which transition is most commonly observed in the UV-Vis spectra of organic molecules with conjugated pi systems?

Accepted Answer

π → π*

Question 10

If the energy gap (ΔE) between the HOMO and LUMO of a molecule decreases, what happens to the wavelength (λ) of light absorbed?

Accepted Answer

The wavelength increases (absorbs at longer wavelengths).

Question 11

Which of the following correctly ranks the types of electromagnetic radiation from lowest to highest energy?

Accepted Answer

Radio \u003c Microwave \u003c Infrared \u003c Visible \u003c Ultraviolet \u003c X-ray \u003c Gamma

Question 12

A solution of a colored compound is diluted by a factor of 2. What happens to its absorbance, assuming all other variables remain constant?

Accepted Answer

The absorbance is halved.

Question 13

Which of the following molecules is most likely to absorb at the longest wavelength in the UV-Vis region?

Accepted Answer

A molecule with four conjugated double bonds

Question 14

A sample has an absorbance of 1.00 at 300 nm in a 1 cm cell. If the molar absorptivity is 2000 L·mol⁻¹·cm⁻¹, what is the concentration of the sample?

Accepted Answer

5.00 × 10⁻⁴ mol·L⁻¹

Question 15

Which of the following best explains why compounds with extended conjugation often appear colored to the human eye?

Accepted Answer

They absorb visible light due to a small HOMO-LUMO gap, and the color observed is the complement of the absorbed wavelength.

Question 16

A UV-Vis spectrum of a compound shows a λmax at 260 nm. Which of the following structural features is most likely present?

Accepted Answer

A conjugated diene system

Question 17

Which of the following is NOT a factor in the Beer-Lambert Law equation for absorbance?

Accepted Answer

Wavelength of light

Question 18

A researcher wants to determine the concentration of a protein solution using UV-Vis spectroscopy. Which property of the protein is most important for this analysis?

Accepted Answer

The presence of conjugated aromatic amino acids that absorb in the UV region

Question 19

If a molecule absorbs light at 400 nm, what is the approximate energy of the absorbed photon? (Use h = 6.63 × 10⁻³⁴ J·s, c = 3.00 × 10⁸ m/s)

Accepted Answer

4.97 × 10⁻¹⁹ J

UV-Visible Spectroscopy and the Electromagnetic Spectrum: Study Notes

Study Guide - Practice Questions

Study Guide - Flashcards