A student records the visible absorption spectrum of compound X over the range 350 nm…
CHEMISTRY · Multi-part · AP · Chemistry · Properties of Substances and Mixtures · Properties of Substances and Mixtures
Problem
A student records the visible absorption spectrum of compound X over the range 350 nm to 650 nm using a 1.00 cm cuvette. The spectrum is verbally summarized below.
• A broad peak is observed, reaching its highest absorbance of 1.20 at 465 nm.
• At 430 nm the absorbance is 0.95, and at 500 nm it is 0.90.
Assume that instrumental baseline noise is negligible compared with the absorbance values reported.
Answer the following questions about quantitative analysis of compound X using this spectrum.
(a) Identify the wavelength that should be selected for routine quantitative measurements of compound X and state its numerical value.
(b) Explain, in terms of molar absorptivity and analytical sensitivity, why the wavelength you selected in part (a) is preferable to 430 nm or 500 nm.
(c) A chemist accidentally sets the instrument to 500 nm while still using the calibration curve prepared at the wavelength chosen in part (a). Predict qualitatively (greater than, less than, or equal to the true value) how the calculated concentration of an unknown sample will differ from the true concentration, and justify your prediction.