▶️Answer/Explanation
Ans: A
Absorption of photons in the visible range will cause transition of electrons between electron energy levels.
Question
The infrared spectrum above represents the absorption of certain wavelengths of radiation by molecules of CO2. Which of the following best explains what occurs at the molecular level as the CO2 molecules absorb photons of the infrared radiation?
A The atoms in the \(CO_2\) molecules increase their vibration as the bonds between the atoms bend and stretch.
B The molecules of \(CO_2\) increase the energy of their rotational motions.
C The electrons in the valence shells of the atoms in the \(CO_2\) molecules are promoted to higher electronic energy levels.
D The bonds between the atoms in the \(CO_2\) molecules are continuously broken and then reform.
▶️Answer/Explanation
Ans: A
The absorption of photons of infrared radiation is associated with transitions in molecular vibrational levels.
▶️Answer/Explanation
Ans:B
The presence of a carbonyl functional group results in an intense peak within a narrow range in an infrared spectrum that corresponds to the carbonyl stretching vibration; thus infrared spectroscopy can be used to confirm that the product contains a carbonyl group.
Question
A student prepared five solutions of\( CuSO_{4}\) with different concentrations, and then filled five cuvettes, each containing one of the solutions. The cuvettes were placed in a spectrophotometer set to the appropriate wavelength for maximum absorbance. The absorbance of each solution was measured and recorded. The student plotted absorbance versus concentration, as shown in the figure above. Which of the following is the most likely explanation for the variance of the data point for the 0.600 M \(CuSO_{4}\) solution?
(A) The cuvette into which the 0.600 M solution was placed had some water droplets inside.
(B) The cuvette into which the 0.600 M solution was placed was filled slightly more than the other cuvettes.
(C) The wavelength setting was accidentally moved away from that of maximum absorbance.
(D) The cuvette used for the 0.600 M solution had not been wiped clean before being put in the spectrophotometer.
▶️Answer/Explanation
Ans:D
Refer to the following information.
\(Cu(s)+4HNO_{3}(aq)\rightarrow Cu(NO_{3})(aq)+NO_{2}(g)+2H_{2}O(l)\)
Each student in a class placed a 2.00 g sample of a mixture of Cu and Al in a beaker and placed the beaker in a fume hood. The students slowly poured 15.0 mL of 15.8 M HNO3(aq) into their beakers. The reaction between the copper in the mixture and the HNO3(aq) is represented by the equation above. The students observed that a brown gas was released from the beakers and that the solutions turned blue, indicating the formation of \(Cu^{2+}\)(aq). The solutions were then diluted with distilled water to known volumes.
Question
To determine the number of moles of Cu in the sample of the mixture, the students measured the absorbance of known concentrations of Cu\((NO_{3})_{2}\)(aq) using a spectrophotometer. A cuvette filled with some of the solution produced from the sample of the mixture was also tested. The data recorded by one student are shown in the table above. On the basis of the data provided, which of the following is a possible error that the student made?
(A) The Cu\((NO_{3})_{2}\)(aq) from the sample of the mixture was not diluted properly.
(B) The spectrophotometer was calibrated with tap water instead of distilled water.
(C) The student labeled the cuvettes incorrectly, reversing the labels on two of the solutions of known concentration.
(D) The spectrophotometer was originally set to an inappropriate wavelength, causing the absorbance to vary unpredictably.
▶️Answer/Explanation
Ans:B