Question

A student prepares a caffeine standard solution that has a concentration of 1858 ppm. To prepare a calibration curve, the student pipets various volumes of the standard solution into 50.0 mL volumetric flasks and dilutes to the fill line with water. The absorbance of each solution is measured in a 1.00 cm cuvette using the spectrophotometer and is shown in the table below. Calculate the concentration of each solution and create a calibration curve using the absorbance data provided. Based on this data, what is the molar absorptivity of caffeine at this wavelength? (Enter your numerical answer in decimal format to three significant figures. Do not enter units)

Volume Caffiene Std Sln (mL)	Absorbance Value
0.125	0.17717
0.270	0.35923
0.390	0.44581
0.500	0.62144
0.950	1.2056

HINT - Calculate the concentrations of caffeine (in ppm) of each solution - these are your "x" values. Absorbance values are on the "y" axis. Perform a linear regression and find the slope. There are videos on using Excel posted to Blackboard.

Answer 1

Ans.

Final vol. ml Abs V2 y = 0.0337x + 0.0023 R?=0.9953 Blank Stock Caf) Vol. of stock ppm mL C1 V1 1858 0.125 1858 0.270 1858 0.390 1858 0.500 1858 0.950 50.00 50.00 50.00 50.00 50.00 Final Caf] in aliquots (ppm) C2 = (C1V1/V2) 4.645 10.033 14.492 18.580 35.302 0.17717 0.35923 0.44581 0.62144 1.2056 w Abs Slope Absorption coefficient, E = Path length Given (from the graph) - Slope = 0.0337 ppm Path length = 1.00 cm #Putting the values in above expression - 0.0337 M-1 0.0337 ppm-cm-1 1.00 cm 1.735E-07 M-1 cm -5.000 0.000 5.000 10.000 15.000 20.000 25.000 30.000 35.000 40.000 -02 (Caffeine], ppm ppm Molarity = - Equation 2 1000 x MW