Question

If under the student's experimental conditions, the actual value of K, for this equilibrium is 45. From the statements below, identify the possible source(s) of error for this student's trial.

a) To calibrate the spectrophotometer, water was used as a blank.
b) To calibrate the spectrophotometer, 0.750 M Fe(NO3)3 solution was used as a blank.
c) Finger prints on the cuvette containing the Fe(SCN)2+ solution were not wiped off before collecting the absorbance.
d) There was an error extrapolating the calibration curve, where the experimentally determined molar absorptivity coefficient is smaller than its true value.
e) None of the statements listed would contribute to the error in this student's trial.

Answer 1

The possible sources of error for the student's trial are using water as a blank in the spectrophotometer calibration, not wiping off fingerprints on the cuvette and an error in extrapolating the calibration curve.

Step-by-step explanation:

The possible source(s) of error for the student's trial are:

1. To calibrate the spectrophotometer, water was used as a blank. This could introduce inaccuracies in the absorbance readings.
2. Fingerprints on the cuvette containing the Fe(SCN)2+ solution were not wiped off before collecting the absorbance. This could affect the accuracy of the absorbance measurements, as fingerprints can interfere with light transmission.
3. There was an error extrapolating the calibration curve, where the experimentally determined molar absorptivity coefficient is smaller than its true value. This indicates that the spectrophotometer measurements may not be correctly calibrated.

Answer 2

Several potential sources of error could impact the accuracy of the experimentally determined equilibrium constant, including improper calibration of the spectrophotometer, fingerprints on the cuvette, and errors in the extrapolation of the calibration curve. So the correct options are a,b,c, and d.

Step-by-step explanation:

When considering the possible sources of error in the student's experimental determination of an equilibrium constant K, several factors could lead to inaccuracies:

• (a) Using water as a blank to calibrate the spectrophotometer could introduce errors if the water has impurities or absorbance differences compared to the solvent used in the reaction mixture. The blank should match the solvent of the reaction mixture as closely as possible.
• (b) Using a 0.750 M Fe(NO3)3 solution as a blank when calibrating the spectrophotometer will impact the absorbance readings, as this solution has a different absorbance profile than the blank that should have been used, possibly the solvent alone or a matrix-matched blank.
• (c) Fingerprints on the cuvette containing the Fe(SCN)2+ solution could distort absorbance measurements by scattering light or adding additional absorbance, causing inaccuracies in the determination of the equilibrium concentration.
• (d) An error in extrapolating the calibration curve could mean that the experimentally determined molar absorptivity is incorrect. If it is underrepresented, this could lead to an incorrect calculation of concentrations, and thus an erroneous equilibrium constant.

Options (a), (b), (c), and (d) are all potential sources of error that could affect the accuracy of the student's determination of K. These errors could lead to an incorrect experimentally determined equilibrium constant, which should be addressed to ensure accuracy in the experiment results.