A research team is evaluating the reliability of a student’s spectrophotometric…
CHEMISTRY · Multi-part · AP · Chemistry · Properties of Substances and Mixtures · Properties of Substances and Mixtures
Problem
A research team is evaluating the reliability of a student’s spectrophotometric experiment designed to determine the concentration of an aqueous dye solution. A series of four 1:1 (v:v) serial dilutions of the original stock was prepared in 10.0 mL volumetric flasks, and the absorbance of each solution was measured at the same wavelength using a 1.00 cm cuvette. The student obtained the following qualitative observations:
• The color of each successive solution became progressively lighter.
• The absorbance values decreased nearly by half with each dilution.
• A slight systematic offset was noticed when the student compared class data collected with different pipettes.
Use these observations to discuss the underlying chemical and instrumental principles.
(a) Describe the fundamental relationship that connects the intensity of color in a solution to its concentration, and explain how this relationship accounts for the student’s observation that each serial dilution produced a noticeably lighter solution.
(b) The student noticed that the absorbance value of each dilution was approximately one-half that of the preceding solution. Explain quantitatively why this trend is expected for a sequence of 1:1 serial dilutions.
(c) Suppose the pipette used to transfer the dye stock consistently delivered 0.05 mL less than its nominal 5.00 mL volume. Explain how this systematic error would affect the calculated concentration of the original stock if the student determined the stock concentration from the absorbance of the fourth dilution.