Question

In the Spectrophotometric Analysis of Aspirin lab, a student with poor technique overfills the 250.00 mL volumetric flask when making up the standard solution. The meniscus ends up well above the line on the neck of the flask when the delonized water is added to the acetylsalicylic acid in the flask. Instead of starting over, the student pours out some of the solution in the flask so that the bottom of the meniscus is below the line. The student then uses a pipette to add enough deionized water to fill the flask so that the bottom of the meniscus is right on the line. Despite this error, the student is confident that the directions have been followed correctly, and completes the experiment and calculations with no further changes. What effect will this have on the graph of absorbance vs concentration? (Hint: first determine if there is an effect on the concentrations of the standards and then determine how that error may manifest) in the graph shown below, the line with blue diamond data points is the correct line using data from a student with excellent technique. The line made with data from the poor technique will be: (choose one of the 4 answers below) Absorbance versus Concentration Absorbance 20 30 90 100 110 40 50 60 70 80 Concentration (mg acetylsalicylic acid per L.) A. artificially shifted upward as in the line with the pink box data points B. artificially shifted downward as in the line with the yellow triangle data points C. unaffected, as in the line with the blue diamond data points D. You are unable to tell how the line will be affected.

Answer 1

According to the Beer's law, the absorbance is directly proportional to the concentration. In this case, the student has added excess water to the flask. This will mean that the real concentration will be lower than what the student records on his experiment. Now, the absorbance will obviously be dependent on the real concentration and not on the recorded concentration. This means that according to the Beer's law, the absorbance for every data point will decrease because the real concentration has decreased. Thus, for every recorded concentration, the points on the graph will shift downwards and the student will get the yellow triangle line. So, the answer will be B) Artificially shifted downwards as in the line with the yellow triangle data points.

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