An instructor provides the following calibration data for a colored compound measured…
CHEMISTRY · Multi-part · AP · Chemistry · Properties of Substances and Mixtures · Properties of Substances and Mixtures
Problem
An instructor provides the following calibration data for a colored compound measured spectrophotometrically at its maximum‐absorbance wavelength in a 1.00 cm cuvette.
Concentration (M): 1.00×10−4, 2.00×10−4, 3.00×10−4, 4.00×10−4
Absorbance: 0.151, 0.303, 0.455, 0.603
A student analyzes an unknown that undergoes the two-step dilution sequence described below before its absorbance is measured.
Step 1: A 2.00 mL aliquot of the unknown is diluted to 50.0 mL in a volumetric flask.
Step 2: A 10.0 mL portion of the Step 1 solution is further diluted to 100.0 mL.
The absorbance of the Step 2 solution is 0.315 at the same wavelength.
(a) Use the calibration data to determine the proportionality constant that relates absorbance to concentration for this compound. Show your method clearly.
(b) Calculate the molarity of the compound in the Step 2 solution from its measured absorbance.
(c) Determine the molarity of the original (undiluted) unknown solution, accounting for the two serial dilution steps.
(d) The certified reference concentration for this compound is 0.0500 M. Calculate the percent error in the student’s result and identify the most plausible source of this error based on the procedures used.