Resumen de Waters of Hydration of Cupric Hydrates: A Comparison between Heating and Absorbance Methods
Rebecca Barlag, Frazier Nyasulu
The empirical formulas of four cupric hydrates are determined by measuring the absorbance in aqueous solution. The Beer−Lambert Law is verified by constructing a calibration curve of absorbance versus known Cu2+(aq) concentration. A solution of the unknown hydrate is prepared by using 0.2−0.3 g of hydrate, and water is added such that the total mass is between 12 and 15 g. The density of the prepared solution is determined using a 1.00 mL autopipettor, and its absorbance measured. From these measurements, the water of hydration is calculated. The procedure conserves materials, produces little waste, and takes ∼15 min per sample; therefore, students are able to analyze the four hydrates in one lab period. The compounds are cupric sulfate pentahydrate, cupric nitrate hemi(pentahydrate), cupric chloride dihydrate, and cupric sulfate (anhydrous). Students also analyze one of these cupric hydrates by heating. The heating approach uses more material and takes longer, but the results from the two approaches are comparable.
