Question

a student is given the task of making a calibration curve for cocl₂(aq) for use in a spectrophotometry experiment. the student pours a sample of cocl₂(aq) of known concentration into a cuvette with a 1.0 cm path length and tries to measure the absorbance at 500 nm, which is the wavelength of maximum absorbance. however, the absorbance is too high to give an accurate reading. which of the following is the most appropriate next step for the student?

a switching to using a cuvette with a 1.5 cm path length

b repeating the experiment after warming the solution to 40°c

c diluting a measured volume of the solution to a new known volume using distilled water

d pouring some solution out of the cuvette

Explanation:

According to Beer - Lambert's Law \(A=\epsilon cl\) (where \(A\) is absorbance, \(\epsilon\) is molar absorptivity, \(c\) is concentration, and \(l\) is path length). To decrease absorbance \(A\) (since the current \(A\) is too high), we can either decrease \(c\) (concentration) or \(l\) (path length).

• Option A: Using a cuvette with a longer path length (\(1.5\ cm\) instead of \(1.0\ cm\)) will increase \(A\) (because \(A\propto l\) from \(A = \epsilon cl\)), so this is incorrect.
• Option B: Warming the solution may change some properties of the solution (like volume due to thermal expansion slightly, but not in a controlled, measurable way for a calibration curve). Also, there is no direct relationship between temperature and absorbance in a simple, predictable manner for making a calibration curve in this context. So this is incorrect.
• Option C: Diluting a measured volume of the solution (using \(M_1V_1 = M_2V_2\) to calculate the new concentration \(M_2\)) will decrease the concentration \(c\). Since \(A=\epsilon cl\) and \(\epsilon\) (for a given substance at a given wavelength) and \(l\) (if we keep the cuvette the same) are constant, decreasing \(c\) will decrease \(A\) in a measurable, reproducible way. This is correct.
• Option D: Pouring some solution out of the cuvette does not change the concentration \(c\) of the solution. The absorbance \(A\) (which depends on \(c\) as \(A=\epsilon cl\)) will remain the same. So this is incorrect.

Answer:

C. Diluting a measured volume of the solution to a new known volume using distilled water