Question

a student prepared five solutions of cuso4 with different concentrations, and then filled five cuvettes, each containing one of the solutions. the cuvettes were placed in a spectrophotometer set to the appropriate wavelength for maximum absorbance. the absorbance of each solution was measured and recorded. the student plotted absorbance versus concentration, as shown in the following is the most likely explanation for the variance of the data point for the
a the cuvette into which the 0.600 m solution was placed had some water droplets inside
b the cuvette into which the 0.600 m solution was placed was filled slightly more than the other cuvettes.
c the wavelength setting was accidentally moved away from that of maximum absorbance.
d the cuvette used for the 0.600 m solution had not been wiped clean before being put in the spectrophotometer.

Explanation:

• Option A: If the cuvette for the \(0.600\ M\) solution had water droplets inside, it would dilute the solution. According to Beer - Lambert's law \(A=\epsilon lc\) (where \(A\) is absorbance, \(\epsilon\) is the molar absorptivity, \(l\) is the path length, and \(c\) is the concentration), a lower - than - expected concentration would result in a lower absorbance. But the data point for \(0.600\ M\) has an absorbance higher than the trend line. So this option is incorrect.
• Option B: The Beer - Lambert's law is based on the assumption that the path length \(l\) is constant. If the cuvette was filled slightly more, the path length \(l\) (in a standard spectrophotometer, the light passes through a fixed - width cell, and filling more does not change the effective path length that the light travels through the solution in the direction perpendicular to the light beam) does not change in a way that would affect the absorbance in the context of the law (\(A=\epsilon lc\)). So this option is incorrect.
• Option C: If the wavelength setting was moved away from the maximum absorbance, the molar absorptivity \(\epsilon\) (which is wavelength - dependent) would decrease. According to \(A = \epsilon lc\), a lower \(\epsilon\) would result in a lower absorbance. But the data point for \(0.600\ M\) has a higher absorbance. So this option is incorrect.
• Option D: If the cuvette was not wiped clean, there would be smudges or residues on the outside of the cuvette. These would scatter the light. Scattered light makes the detector receive less of the transmitted light, and the instrument (assuming it is calibrated to take absorbance as \(A=-\log(\frac{I}{I_0})\), where \(I\) is the transmitted light intensity and \(I_0\) is the incident light intensity) would register a higher absorbance (because \(I\) is lower). This is consistent with the data point for \(0.600\ M\) having a higher absorbance than the trend line.

Answer:

D. The cuvette used for the \(0.600\ M\) solution had not been wiped clean before being put in the spectrophotometer.