A student wants to determine the concentration of permanganate, MnO4− (aq), in a solution. The student plans to use colorimetric analysis because solutions containing MnO4− (aq) have a purple color.
To determine the optimum wavelength for an experiment that measures the concentration of MnO4− (aq), the student takes a sample of the solution and measures the amount of light absorbed by the sample over a range of wavelengths. The data are plotted in the graph shown. Identify the optimum wavelength that the student should use for the experimental procedure.
The student uses a stock solution of 2.40 × 10−3 M KMnO4 (aq) to prepare the standard solutions of MnO4− (aq) that are needed to construct a calibration curve.
i) The student uses a 100.0 mL graduated cylinder to measure a certain volume of KMnO4 (aq) stock solution, as shown in the diagram given. What volume should the student record?
ii) Calculate the volume, in mL, of 2.40 × 10−3 M KMnO4 (aq) that is required to produce 100.0 mL of a standard 1.68 × 10−3 M MnO4− (aq) solution.
The student designs the following procedure to produce a calibration curve.
Step 1: Prepare several standard solutions that have known MnO4− (aq) concentrations by dilution of the stock solution.
Step 2: Rinse the cuvette with distilled water.
Step 3: Rinse the cuvette with the standard solution and fill the cuvette with the standard solution.
Step 4: Measure the absorbance of the standard solution with the colorimeter.
Step 5: Repeat steps 2-4 for each of the standard solutions.
The data are plotted in the calibration curve shown. One of the data points (indicated with an arrow) on the calibration curve is below the line of best fit.
Assuming that all lab equipment is functioning properly, identify which one of the procedural steps the student could have executed incorrectly that would explain why the marked data point is below the line of best fit. Justify your answer.
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