Error Analysis Questions for Photometry Experiment 1. In preparation for constru

Question

Error Analysis Questions for Photometry Experiment 1. In preparation for constructing a calibration curve, a student consistently adds too much water and overshoots the marking on the volumetric flask while diluting the stock solution of potassium permanganate. Given this systematic error, would the determination of the unknown concentration be overestimated, underestimated or remain unaffected? Explain your reasoning. 2. When transferring the unknown solution from a test tube to the cuvette used in the spectrophotometer, a student spills some of the liquid out of the cuvette. Would this result in the unknown concentration being overestimated, underestimated or remain unaffected? Explain your reasoning. 3. Imagine the unknown solution was transferred into a cuvette that wasn't dry and contained droplets of deionized water. Would this result in the unknown concentration being overestimated, underestimated or remain unaffected? Explain your reasoning

Explanation / Answer

1) The student added water beyond the graduation mark while preparing the standard solutions; therefore, the volume of the solution was higher than desired.

We define the concentration of a substance as

concentration = (moles of the substance)/(volume of the solution)

Since the volume of the solution was higher than desired, the concentrations of the standards will be lower than the actual values. Hence, the calibration curve will have a bias toward lower values. The concentration of the unknown sample will therefore, we underestimated, since the calibration curve had an intrinsic bias toward lower values.

2) It is common practice that the cuvette is half filled with the sample solution during analysis. The absorbance of light by a sample depends only on the concentration of the sample and the path length of the cuvette. Since, both remain constant here, it is immaterial if the student spills some sample while filling the cuvette. The concentration of the unknown sample will remain unaltered.

3) If the cuvette is not completely dry, the water inside the cuvette will dilute the sample solution; hence, the concentration of the sample solution decreases (since the number of moles remain constant, but the volume increases due to dilution). Hence, the concentration of the unknown will be underestimated.

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