Answer to Question #347241 in General Chemistry for Diekis

Question

“determine the concentration of an unknown Copper Sulphate Solution: by measuring its absorbance and using the slope of the Beer’s Law Curve”.

Answer 1

You will be using the Colorimeter shown in Figure 1. In this device, light from an LED light source will pass through the solution and strike a photocell. The CuSO₄ solution used in this experiment has a deep blue color. A higher concentration of a colored solution will absorb more light and transmit less light than a solution of lower concentration. The colorimeter will interpret the light received by the photocell and express it both as an absorbance and a percent transmittance value.

Picture 1.

Picture 2.

You will prepare five copper (II) sulfate solutions of known concentration (standard solutions). Each solution will be transferred to a small, rectangular cuvette that is placed into the colorimeter. The absorbance of each solution will be measured and recorded. When a graph of absorbance vs. concentration is plotted for the standard solutions, a direct relationship should result, as shown in Figure 2. This direct relationship is known as Beer's Law, and is shown by the equation

A = εcl

where A is the absorbance measured with a colorimeter, ε is the molar absorptivity, a constant for the particular solute you are analyzing, c is the molar concentration of the solute, and l is path length, the distance light travels through the cuvette (usually 1 cm). For a single solute, absorbance and concentration are directly proportional if the path length is constant. When a linear trendline analysis is performed on a graph of absorbance vs. concentration, the slope is equal to the molar absorptivity, ε, if the path length is 1 cm. The concentration of an unknown CuSO₄ solution will then be determined by measuring its absorbance with the colorimeter and using the molar absorptivity from your trendline equation to determine its concentration.