This research details a comparison of two methods used by students in a General Chemistry laboratory course for determining the amount of copper in a solid sample. Isolation of the copper from the original sample, solid copper (II) chloride dihydrate (37.274% copper), was performed in the same manner for each method. After dissolution of the original copper salt in acidic solution, elemental copper was precipitated using solid magnesium and separated from aqueous solution via filtration. For both methods, magnesium turnings were added to a teal copper (II) chloride solution until it was determined, as indicated below, that all the copper had precipitated from solution. Afterwards, excess magnesium was reacted with concentrated hydrochloric acid. The solution was then filtered and the precipitate was washed, dried, and weighed. The two methods varied in the determination of when the removal of the copper from solution was complete. In one method, a solution color change from teal to colorless was used as indication of a completed precipitation of copper. In the alternative method, ammonia was added to aliquots of solution which indicated remaining copper ions with a royal blue complex. After 35 groups of students performed the analysis with one of the two methods, the average percent copper obtained with and without ammonia testing was (29±2)% and (23±3)% respectively within a 95% confidence interval. An F-test indicated unequal variances between the two methods. The corresponding t-test value was 3.34 compared to the two-tailed t-critical value of 2.04, indicating a statistically significant variation of data between the two methods.

Improving analyses for general chemistry laboratory students: Comparison of methods for the determination of percentage copper in a solid sample

2019 Spring National Meeting

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