Q: If a sample has an incident light intensity $$I_0 = 100$ and a transmitted light intensity I_t = 25$, what is the percent transmittance (%T)?

$$\%T = \frac{I_t}{I_0$} \times 100 = 25\%$$

Question 1

Which of the following best describes the relationship between absorbance (A) and transmittance (T) as measured by a spectrophotometer?

Accepted Answer

$$A = -\log_{10}(T)$$

Question 2

A solution transmits 40% of the incident light at a particular wavelength. What is the absorbance (A) of this solution?

Accepted Answer

Question 3

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

Accepted Answer

Absorbance is directly proportional to both the concentration of the absorbing substance and the path length of the sample.

Question 4

A spectrophotometer is set to measure absorbance at 625 nm. If a blank solution is used, what is its primary purpose?

Accepted Answer

To account for absorbance due to the solvent and cuvette, ensuring only the sample's absorbance is measured.

Question 5

If a sample has an incident light intensity $$I_0 = 100$ and a transmitted light intensity I_t = 25$, what is the percent transmittance (%T)?

Accepted Answer

$$\%T = \frac{I_t}{I_0$} 	imes 100 = 25\%$$

Question 6

Which of the following best explains why a green pigment appears green to our eyes?

Accepted Answer

It absorbs most wavelengths except green, which it reflects or transmits.

Question 7

A student prepares a series of DCPIP solutions of increasing concentration and measures their absorbance at 625 nm. What type of graph should the student expect if the Beer-Lambert Law holds?

Accepted Answer

A straight line with a positive slope when plotting absorbance versus concentration.

Question 8

Which of the following is NOT a correct step when using a spectrophotometer to measure absorbance?

Accepted Answer

Measuring absorbance directly after inserting the sample without using a blank.

Question 9

If a solution has an absorbance of 0.30 at a certain wavelength, what is its transmittance (T)?

Accepted Answer

$$T = 10$$^{-A}$$ = 10$$^{-0.30}$$ \approx 0.50$$

Question 10

Which of the following best describes the function of the monochromator (prism or grating) in a spectrophotometer?

Accepted Answer

It isolates a single wavelength of light from the light source.

Question 11

A student accidentally uses a cuvette with fingerprints on the clear sides. How will this most likely affect the absorbance reading?

Accepted Answer

The absorbance reading will be artificially high due to additional scattering and absorption by the fingerprints.

Question 12

Why is it important to use a blank solution that is identical to the sample solution in all aspects except for the analyte?

Accepted Answer

To correct for absorbance due to the solvent and cuvette, ensuring only the analyte's absorbance is measured.

Question 13

If a spectrophotometer is not zeroed with a blank before measuring a sample, what is the most likely consequence?

Accepted Answer

The measured absorbance will include contributions from the solvent and cuvette, leading to inaccurate results.

Question 14

Which of the following is a correct interpretation of the Beer-Lambert Law equation $$A = \varepsilon c l$$?

Accepted Answer

Absorbance increases as either the concentration ($$c) or $$path length ($$l) $$increases, assuming molar absorptivity ($$\varepsilon) is $$constant.

Question 15

A solution of DCPIP is measured at 625 nm and gives an absorbance of 0.50. If the path length is 1 cm and the molar absorptivity ($$\varepsilon) is$$ $$10,000\ \mathrm{L\ $$mol^{-1}$$\ cm$$^{-1}$$}$$, what is the concentration ($$c) of $$DCPIP in mol/L?

Accepted Answer

$$c = \frac{A}{\varepsilon l} = \frac{0.50}{10,000 	imes 1} = 5.0 	imes 10$$^{-5}$$\ \mathrm{mol/L}$$

Question 16

Which of the following best explains why cloudy or opaque samples cannot be analyzed in standard spectrophotometers?

Accepted Answer

They scatter light, preventing accurate measurement of transmitted light intensity.

Question 17

If a spectrophotometer is set to a wavelength outside the absorbance maximum of a pigment, what is the most likely result?

Accepted Answer

The measured absorbance will be lower than at the absorbance maximum.

Question 18

A student is preparing a standard curve for DCPIP at 625 nm. Which of the following is the correct procedure?

Accepted Answer

Measure the absorbance of solutions with known concentrations and plot absorbance versus concentration.

Question 19

Which of the following is a real-world application of spectrophotometry?

Accepted Answer

Measuring the concentration of hemoglobin in blood samples.

Spectrophotometry and Absorption by Pigments: Principles and Laboratory Applications Lab 15

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