Question 37 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.

37. If not enough oxidant is added to convert all the Mn⁺² to MnO₄^- in Experiment 2, how would the value of the absorbance be affected? The value of the absorbance would be:

A. the same as if all the Mn⁺²is converted, because the same mass of Mn is present.

B. the same as if all the Mn⁺²is converted, because the wavelength is the same.

C. less than if all the Mn⁺²is converted.

D. greater than if all the Mn⁺² is converted.

Answer and Explanation

Your Answer is

Correct Answer is C

Explanation

As we said in the previous question, permanganate is the key to absorbing light and testing the concentration. If the concentration is low, the absorbed light will naturally be less, and if the oxidant is not added enough, there will be less light-absorbing substances (permanganate)