Q: A solution in a 1 cm cuvette has an absorbance of 0.500 at a certain wavelength. If the molar absorptivity $$\epsilon is$$ $$2000\ \mathrm{M}^{-1}\mathrm{cm}^{-1}$$, what is the concentration of the absorbing species?

$$2.5 \times 10$$^{-4}$$\ \mathrm{M}$$

Question 1

Which equation correctly represents Beer's Law for absorbance in spectrophotometry?

Accepted Answer

$$A = \epsilon c b$$

Question 2

A solution in a 1 cm cuvette has an absorbance of 0.500 at a certain wavelength. If the molar absorptivity $$\epsilon is$$ $$2000\ \mathrm{M}^{-1}\mathrm{cm}^{-1}$$, what is the concentration of the absorbing species?

Accepted Answer

$$2.5 	imes 10$$^{-4}$$\ \mathrm{M}$$

Question 3

Which component in a spectrophotometer is responsible for isolating a narrow wavelength region from the light source?

Accepted Answer

Monochromator

Question 4

A spectrophotometer measures the intensity of light entering ($$P_0) $$and emerging ($$P) $$from a sample. If $$P_0 = 1.00$ and P = 0.10$, what is the absorbance?

Accepted Answer

$$A = 1.00$$

Question 5

Which light source is most suitable for measurements in the UV region (160–380 nm)?

Accepted Answer

Deuterium lamp

Question 6

Why do molecular absorption bands appear broader than atomic absorption lines in UV-Visible spectroscopy?

Accepted Answer

Molecules have quantized vibrational and rotational energy levels in addition to electronic transitions.

Question 7

A grating monochromator with 1450 grooves/mm is irradiated at an incident angle of $$48^\circ. $$What is the formula to calculate the wavelength ($$\lambda) $$that appears at a reflection angle of $$20^\circ$$?

Accepted Answer

$$\lambda = d(\sin 48^\circ - \sin 20^\circ)$$

Question 8

Which of the following statements about lasers as light sources in spectroscopy is TRUE?

Accepted Answer

Lasers require population inversion to operate.

Question 9

In a double-beam spectrophotometer, what is the main advantage over a single-beam design?

Accepted Answer

It allows simultaneous measurement of sample and reference, improving baseline stability.

Question 10

Which detector is commonly used in UV-Visible spectrophotometry to convert light into an electrical signal?

Accepted Answer

Photomultiplier tube

Question 11

If the transmittance ($$T) of a $$solution is 25%, what is its absorbance ($$A$$)?

Accepted Answer

$$A = -\log_{10}(0.25) = 0.60$$

Question 12

Which of the following best describes the role of a monochromator in a spectrophotometer?

Accepted Answer

It isolates a narrow band of wavelengths from the light source.

Question 13

What is the relationship between the number of spectral lines ($$n) $$and the spin quantum number ($$s) $$for singlet and triplet states?

Accepted Answer

$$n = 2s + 1$$

Question 14

Which transition in formaldehyde corresponds to the absorption at 355 nm?

Accepted Answer

Singlet $$n \rightarrow \pi^*$$

Question 15

Why is the slit width in a monochromator important for spectrophotometric measurements?

Accepted Answer

A narrower slit provides better adherence to Beer's Law and improved sensitivity.

Question 16

Which of the following is TRUE about Fourier Transform Spectroscopy?

Accepted Answer

It acquires the entire spectrum simultaneously and transforms it into its component frequencies.

Question 17

A photomultiplier tube increases sensitivity by:

Accepted Answer

Using multiple dynodes to amplify the electron signal.

Question 18

Which of the following is a correct statement about electronic transitions in molecules?

Accepted Answer

Electronic transitions involve movement of electrons from occupied to unoccupied orbitals.

Question 19

Which of the following is NOT a typical component of a spectrophotometer?

Accepted Answer

Electrochemical cell

Question 20

If a monochromator exit slit is set too wide compared to the width of the absorption peak, what is the likely consequence?

Accepted Answer

The measured absorbance will be less accurate due to inclusion of unwanted wavelengths.

Spectrophotometry and Electronic Transitions: Analytical Chemistry Study Notes

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