A student prepared five solutions of CuSO4 with different concentrations, and then filled five cuvet — Solutions Chemistry Question

STEPS:

1. Understand Beer's Law and its graphical representation: According to Beer's Law, absorbance ($A$) is directly proportional to the concentration ($c$) of the absorbing species:
$$A = abc$$
where $a$ is the molar absorptivity and $b$ is the path length. On a calibration plot of absorbance versus concentration, this direct relationship should yield a straight, linear best-fit line passing through the origin.
2. Observe the deviation of the data point on the graph: By examining the provided calibration curve, the data point corresponding to the $0.600\text{ M}$ solution lies significantly above the best-fit line. This indicates that the spectrophotometer recorded an abnormally high absorbance reading for this sample compared to its actual concentration.
3. Understand how a spectrophotometer calculates absorbance: A spectrophotometer measures absorbance by comparing the intensity of light entering the sample ($I_0$) to the intensity of light that emerges ($I$) and hits the detector. The instrument calculates absorbance based on how much light is blocked:
$$A = -\log\left(\frac{I}{I_0}\right)$$
Consequently, any physical factor that scatters, blocks, or absorbs light—other than the dissolved $\text{CuSO}_4$ molecules themselves—will reduce the amount of light reaching the detector, causing the machine to calculate an artificially inflated absorbance value.
4. Identify the cause of the elevated absorbance: If the outside of the cuvette was not wiped clean, contaminants on the outer walls (such as fingerprints, grease smudges, dust, or water droplets) will scatter or absorb a portion of the incoming light beam. Because less light reaches the detector, the spectrophotometer records an artificially high absorbance value, causing the plotted data point to deviate upward above the expected linear calibration line. This confirms Option D as the correct explanation.

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WHY_OTHERS_WRONG:

• Option A is incorrect: If there were water droplets inside the cuvette before filling it, they would dilute the $0.600\text{ M}$ copper sulfate solution. This dilution would decrease the concentration ($c$) of $\text{Cu}^{2+}$ ions, which according to Beer's Law would result in a lower absorbance reading (the data point would lie *below* the line), not a higher one.
• Option B is incorrect: In a spectrophotometer, the light beam passes horizontally through the lower portion of the cuvette. As long as the cuvette is filled past the height of the light beam path, filling it slightly higher does not alter the concentration or the horizontal path length ($b$). Thus, it has no effect on the measured absorbance.
• Option C is incorrect: If the wavelength was accidentally shifted away from the wavelength of maximum absorbance, the molar absorptivity ($a$) of the $\text{Cu}^{2+}$ ions would decrease. According to Beer's Law, a lower molar absorptivity results in a lower absorbance reading (the data point would lie *below* the line), which contradicts the observed upward deviation.