Q: A solution of a drug has an absorbance of 0.75 at 310 nm in a 1 cm pathlength cell. If the molar extinction coefficient ($$\varepsilon) is$$ $$15,000\ \mathrm{L\ $$mol^{-1}$$\ cm$$^{-1}$$}$$, what is the concentration of the drug in mol/L?

$$5.0 \times 10$$^{-5}$$\ \mathrm{mol/L}$$

Question 1

Which of the following best describes the principle of UV-Vis spectroscopy in pharmaceutical analysis?

Accepted Answer

It quantifies the absorption of ultraviolet and visible light by molecules, causing electronic transitions.

Question 2

A solution of a drug has an absorbance of 0.75 at 310 nm in a 1 cm pathlength cell. If the molar extinction coefficient ($$\varepsilon) is$$ $$15,000\ \mathrm{L\ $$mol^{-1}$$\ cm$$^{-1}$$}$$, what is the concentration of the drug in mol/L?

Accepted Answer

$$5.0 	imes 10$$^{-5}$$\ \mathrm{mol/L}$$

Question 3

Which of the following transitions is most likely to occur at the shortest wavelength (highest energy) in UV-Vis spectroscopy?

Accepted Answer

$$\sigma \rightarrow \sigma^*$$

Question 4

A bathochromic shift in a UV-Vis spectrum refers to:

Accepted Answer

A shift of the absorption maximum to a longer wavelength (lower energy)

Question 5

Which of the following is NOT a typical cause of a bathochromic shift in a molecule's UV-Vis spectrum?

Accepted Answer

Decreasing the pathlength of the cuvette

Question 6

Why is quartz preferred over glass for UV-Vis cuvettes when measuring in the UV region?

Accepted Answer

Quartz does not absorb in the UV region, while glass does.

Question 7

A pharmaceutical scientist observes a hyperchromic effect after changing the pH of a solution. What does this indicate?

Accepted Answer

The absorption intensity has increased.

Question 8

Which of the following best explains why highly conjugated molecules often appear colored?

Accepted Answer

They absorb light in the visible region due to lower energy electronic transitions.

Question 9

The Beer-Lambert law is given by $$A = \varepsilon l c. $$Which of the following changes will NOT affect the absorbance ($$A) of a $$solution?

Accepted Answer

Changing the solvent to one with identical transparency and no interaction with the analyte

Question 10

A solution of phenylephrine has an absorbance of 1.224 in 0.1 M NaOH and 0.02 in 0.1 M HCl at 292 nm. If its absorbance in buffer at pH 8.5 is 0.349, what is the pKa of the phenolic hydroxyl group? (Use $$pK_a = pH + \log \frac{A_i - A}{A - A_u})$$

Accepted Answer

8.90

Question 11

Which of the following is a limitation of UV-Vis spectroscopy in pharmaceutical analysis?

Accepted Answer

It cannot distinguish between different absorbing species in a mixture.

Question 12

Which light source is typically used for the UV region (180–350 nm) in a UV-Vis spectrophotometer?

Accepted Answer

Deuterium lamp

Question 13

A student prepares a calibration curve for a drug using UV-Vis spectroscopy. What is the main purpose of this curve?

Accepted Answer

To relate absorbance to concentration for quantifying unknown samples

Question 14

Which of the following statements about the Beer-Lambert law is correct?

Accepted Answer

Absorbance is directly proportional to the concentration of the absorbing species.

Question 15

Which of the following best describes an auxochrome?

Accepted Answer

A group that, when attached to a chromophore, shifts the absorption maximum to a longer wavelength

Question 16

Which of the following is a correct real-world application of UV-Vis spectroscopy in pharmaceutical analysis?

Accepted Answer

Measuring the release of a drug from a formulation

Question 17

Which of the following best explains why the UV region below 185 nm is difficult to measure in practice?

Accepted Answer

Most solvents are opaque below 185 nm.

Question 18

Which of the following is the correct formula for calculating absorbance ($$A) $$from incident ($$I_0) $$and transmitted ($$I_t) $$light intensities?

Accepted Answer

$$A = \log \left(\frac{I_0$}{I_t}\right)$$

UV-Visible Spectroscopy in Analytical Chemistry: Principles, Instrumentation, and Applications

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