05. Analysis of general chemistry student samples for concentration of copper in a percent composition laboratory experiment

Abstract

General chemistry students at the University of North Georgia perform an experiment in which copper is precipitated from copper (II) chloride dihydrate solutions, through the addition of magnesium turnings, in order to calculate the percent composition of copper in the compound. Students allow the reaction to continue until it is apparent, based on the color of solution, that all of the copper was precipitated from solution. When the solution is colorless, students assume no more copper is left in solution. Typically, students obtain a value for the percentage copper in the compound that is much lower than expected. Previous studies have indicated this is because not all copper was precipitated from solution. In order to improve student results, this study was carried out to determine how much copper remained in students’ solutions after filtration. Samples from student’s filtrates were collected from multiple lab sections and then analyzed using a spectronic 20 to measure absorption of light by the samples. A calibration curve, constructed by measuring solutions with a known concentration, was used to calculate the concentration of copper remaining in the filtrates from the measured absorbance values. The average concentration of copper left in the student solutions was .00687 M. This equates to approximately 30 mg of copper and an 8% error in an average sample. The concentration of students’ samples ranged as high as .0379 M (170 mg of copper) which would result in a 50% error for the student.