Question

a student uses a spectrophotometer to analyze a solution of blue food dye. the student first rinses a cuvette with distilled water. then the student adds the blue dye solution to the cuvette, forgetting to rinse the cuvette with the blue dye solution first. the student places the cuvette in the spectrophotometer and measures the absorbance of the solution. assuming that some distilled water droplets were still in the cuvette when the blue dye solution was added, how would the measured absorbance be affected? a the measured absorbance would be too low, because the distilled water left in the cuvette would slightly dilute the solution. b the measured absorbance would be too low, because the distilled water would change the optimum wavelength of absorbance. c the measured absorbance would be too high, because the water would allow more light to pass through the cuvette. d the measured absorbance would be too high, because the distilled water would also absorb some light.

Explanation:

According to the Beer - Lambert law ($A = \epsilon l c$, where $A$ is absorbance, $\epsilon$ is the molar absorptivity, $l$ is the path - length, and $c$ is the concentration of the absorbing species), absorbance is directly proportional to the concentration of the absorbing species. When distilled water droplets are left in the cuvette, they dilute the blue dye solution, reducing the concentration of the blue - dye molecules. Since absorbance is proportional to concentration, a lower concentration leads to a lower absorbance. Distilled water does not change the optimum wavelength of absorbance of the blue dye, and it does not cause more light to pass through in a way that would increase absorbance reading (absorbance is related to the amount of light absorbed by the solute). Also, distilled water absorbs very little visible light compared to the blue dye.

Answer:

A. The measured absorbance would be too low, because the distilled water left in the cuvette would slightly dilute the solution.