An analytical chemist assesses the concentration of a metal-ion indicator in an…
CHEMISTRY · Multi-part · AP · Chemistry · Properties of Substances and Mixtures · Properties of Substances and Mixtures
Problem
An analytical chemist assesses the concentration of a metal-ion indicator in an unknown solution. The procedure involves transferring 10.00 mL of the unknown (using a class A pipette with a tolerance of ±0.02 mL) into a 100.0 mL volumetric flask (tolerance ±0.08 mL) and diluting to the mark. The absorbance of the diluted solution is measured in a 1.00 cm cuvette and recorded as 0.428 (±0.003). The molar absorptivity of the indicator at the chosen wavelength is 1.80×103Lmol−1cm−1.
(a) Calculate the best estimate of the molarity of the indicator in the original unknown solution, assuming nominal glassware volumes.
(b) Using the stated tolerances, determine the maximum and minimum possible molarities that could result from this procedure. Clearly show how each source of uncertainty influences the final concentration.
(c) Express the concentration obtained in part (a) with its absolute uncertainty (± value) based on the combined effect of volumetric tolerances only. Assume absorbance uncertainty is negligible for this step.
(d) Evaluate whether the absorbance uncertainty (±0.003) or the volumetric uncertainties contribute more significantly to the total uncertainty in the calculated concentration. Justify quantitatively.