3.11 - 3.13

3.11 - 3.13

by Ashley Saenz

Multiple Choice

Infrared spectroscopy is a useful tool for scientists who want to investigate the structure of certain molecules. Which of the following best explains what can occur as the result of a molecule absorbing a photon of infrared radiation?

• The energies of infrared photons are in the same range as the energies associated with changes between different electronic energy states in atoms and molecules. Molecules can absorb infrared photons of characteristic wavelengths, thus revealing the energies of electronic transitions within the molecules.

• The energies of infrared photons are in the same range as the energies associated with different vibrational states of chemical bonds. Molecules can absorb infrared photons of characteristic wavelengths, thus revealing the types and strengths of different bonds in the molecules.

• The energies of infrared photons are in the same range as the energies associated with different rotational states of molecules. Molecules can absorb infrared photons of characteristic wavelengths, thus revealing the energies of transition between different rotational energy states of the molecules.

• The energies of infrared photons are in the same range as the total bond energies of bonds within molecules. Chemical bonds can be completely broken as they absorb infrared photons of characteristic wavelengths, thus revealing the energies of the bonds within the molecules.

Multiple Choice

Which statement correctly compares what occurs when molecules absorb photons in the microwave region with what occurs when molecules absorb photons in the infrared region?

• Microwave photons cause the molecules to increase their rotational energy states, whereas infrared photons cause the molecules to increase their vibrational energy states.

• Microwave photons cause electrons in the molecules to increase their electronic energy states, whereas infrared photons cause the molecules to increase their rotational energy states.

• Microwave photons cause the molecules to increase their vibrational energy states, whereas infrared photons cause electrons in the molecules to increase their electronic energy states.

• Microwave photons cause the molecules to increase their rotational energy states, whereas infrared photons cause electrons in the molecules to increase their electronic energy states.

Multiple Choice

Beta-carotene is an organic compound with an orange color. The diagram above shows the ultraviolet spectrum of beta-carotene. Which of the following statements is true about the absorption bands in the spectrum?

• The absorption band between 250 and 320 nm is due to transitions in electronic energy levels, and the absorption band between 380 and 520 nm is due to transitions in molecular vibrational levels.

• The absorption band between 250 and 320 nm is due to transitions in molecular vibrational levels, and the absorption band between 380 and 520 nm is due to transitions in molecular rotational levels.

• The two main absorption bands are associated with transitions in electronic energy levels. The band in the region corresponding to shorter wavelengths shows a lower absorbance than the band in the region corresponding to longer wavelengths.

• The two main absorption bands are associated with transitions in molecular vibrational levels. The band in the region corresponding to shorter wavelengths shows a lower absorbance than the band in the region corresponding to longer wavelengths.

Multiple Choice

The diagram above represents the photoelectric effect for a metal. When the metal surface is exposed to light with increasing frequency and energy of photons, electrons first begin to be ejected from the metal when the energy of the photons is 3.3×10−19 J

Which of the following is closest to the frequency of the light with photon energy of 3.3×10−19 J ?

• $$5.0x10^{-53\ }s^{-1}$$  

• $$5.0x10^{-16\ }s^{-1}$$  

• $$5.0x10^{14\ }s^{-1}$$  

• $$5.0x10^{52\ }s^{-1}$$  

Multiple Choice

A student uses visible spectrophotometry to determine the concentration of CoCl2 (aq) in a sample solution. First the student prepares a set of CoCl2 (aq) solutions of known concentration. Then the student uses a spectrophotometer to determine the absorbance of each of the standard solutions at a wavelength of 510 nm and constructs a standard curve. Finally, the student determines the absorbance of the sample of unknown concentration.

A wavelength of 510 nm corresponds to an approximate frequency of 6×1014s−1. What is the approximate energy of one photon of this light?

• $$9x10^{47\ }J$$

• $$3x10^{17\ }J$$  

• $$5x10^{-7\ }J$$  

• $$4x10^{-19\ }J$$  

Multiple Choice

The diagram above represents the photoelectric effect for a metal. When the metal surface is exposed to light with increasing frequency and energy of photons, electrons first begin to be ejected from the metal when the energy of the photons is 3.3×10−19 J

Using the wavelength information provided above, what is the color of the light?

• RED

  (647-760 nm)

• ORANGE

  (585-647 nm)

• YELLOW

  (575-585 nm )

• BLUE

  (424-491 nm)

Multiple Choice

The diagrams above show the ultraviolet absorption spectra for two compounds. Diagram 1 is the absorption spectrum of pure acetone, a solvent used when preparing solutions for an experiment. Diagram 2 is the absorption spectrum of the solute for which the absorbance needs to be measured to determine its concentration. When the student reads the absorbance of the solution at 280 nm, the result is too high. Which of the following is most likely responsible for the error in the measured absorbance?

• The student added too little solute to the acetone before measuring its absorbance

• The student rinsed the cuvette with the solution before filling the cuvette with the solution

• The student forgot to calibrate the spectrophotometer first by using a cuvette containing only acetone

• The wavelength setting was accidentally changed from 280 nm to 300 nm before the student made the measurement

Multiple Choice

A student uses visible spectrophotometry to determine the concentration of CoCl2(aq) in a sample solution. First the student prepares a set of CoCl2(aq) solutions of known concentration. Then the student uses a spectrophotometer to determine the absorbance of each of the standard solutions at a wavelength of 510 nm and constructs a standard curve. Finally, the student determines the absorbance of the sample of unknown concentration.

The student made the standard curve above. Which of the following most likely caused the error in the point the student plotted at 0.050 M Co2+ (aq) ?

• There was distilled water in the cuvette when the student put the standard solution in it.

• There was a few drops of 0.100M Co2+ (aq) standard solution in the cuvette when the student put the 0.050 M standard solution

• The student used a cuvette with a longer path length than the cuvette used for the other standard solutions.

• The student did not run a blank between the 0.050 M Co2+ (aq) solution and the one before it.

Multiple Choice

Using a spectrophotometer, a student measures the absorbance of four solutions of CuSO4 at a given wavelength. The collected data is given in the table above. Which of the following is the most likely explanation for the discrepant data in trial 4 ?

• The solution was at a lower temperature than the solutions in the other trials

• the measurement was made using a different spectrophotometer that uses a cell with a longer path length

• the solution was saturated and the flow of light through the solution was restricted

• the concentration of the solution was actually lower than 0.150M.