Answer to Question #167245 in General Chemistry for Dorina Nunes Da Costa

Question

Describe how colorimetry can be used to determine the concentration of an unknown copper II sulphate solution.

Answer 1

In colorimetry red light from the LED light source will pass through the solution and strike a photocell. The CuSO₄ solution used in this experiment is blue. A higher concentration of the colored solution absorbs more light (and transmits less) than a solution of lower concentration. The colorimeter monitors the light received by the photocell as percent transmittance. 

You will prepare five copper(II) sulfate solutions of known concentration (standard solutions). Each solution is transferred to a small, rectangular cuvette that is placed into the colorimeter. The amount of light that penetrates the solution and strikes the photocell is used to compute the absorbance of each solution. When you graph absorbance vs. concentration for the standard solutions, a direct relationship should result. The direct relationship between absorbance and concentration for a solution is known as Beer’s law.

You will determine the concentration of an unknown CuSO₄ solution by measuring its absorbance with the colorimeter. By locating the absorbance of the unknown on the vertical axis of the graph, the corresponding concentration can be found on the horizontal axis. The concentration of the unknown can also be found using the slope of the Beer’s law curve.