Question 40 Answer and Explanation _ACT 2005 Form 56A Science Section

Passage VII

Solutions containing the purple permanganate ion (MnO₄^-) absorb visible light. The amount of light absorbed can be measured by a spectrometer. White visible light contains light of all colors, and each color is characterized by its wavelength. When light passes through a colored solution only certain colors or wavelengths are absorbed. The absorbance measured varies with each wavelength and also depends on the amount of MnO₄^- present in the solution. Absorbance readings are always adjusted for the absorbance of the blank (a solution prepared in the same way as the test solution except it does not contain MnO₄^-).

Experiment 1

Four standard solutions (solutions containing known concentrations of MnO₄^-) were prepared by dissolving a permanganate compound in acid. They were analyzed by a spectrometer at a wavelength of 530 nm (nanometers). The results are in Figure 1.

Experiment 2

Five kinds of steel pipe (A-E), all containing 0.2 to 2.0% of manganese metal (Mn), were obtained.Equal sections of each pipe were cut and their masses determined. The samples were dissolved in acid to react with the metal to form colorless manganese ion (Mn⁺²). An oxidant was added to chemically react with the Mn⁺² and convert it to MnO₄^-. The solutions were then diluted with water to a total volume of 1 liter (L). The resulting test solutions were analyzed at the same wavelength used in Experiment 1. The results are in Table 1. A blank was prepared using a steel pipe containing no Mn and analyzed in the same manner. The blank had an absorbance of 0.015.

40. Copper (Cu) forms a blue solution of Cu⁺² ions when dissolved in acid. Blue solutions absorb light at 675 nm. What changes in procedure would be necessary if a metal pipe containing Cu rather than Mn was analyzed using a spectrometer?

F. The spectrometer would be set at 530 nm for the standard solutions containing Cu⁺²ions and for the metal pipe test solution.

G. The spectrometer would be set at 530 nm for the standard solutions containing Cu⁺² ions and at 675 nm for the metal pipe test solution.

H. The spectrometer would be set at 675 nm for the standard solutions containing Cu⁺² ions and at 530 nm for the metal pipe test solution.

J. The spectrometer would be set at 675 nm for the standard solutions containing Cu⁺² ions and for the metal pipe test solution.

Answer and Explanation

Your Answer is

Correct Answer is J

Explanation

Experimental questions, when doing experiments, we generally use the most obvious indicators to observe the results. If the precipitate is blue, then we should test the absorbance of blue to get the most obvious data, otherwise testing other colors may give inaccurate or incorrect results.