How Much is a Penny Worth? Percentage Copper in a U.S. One-Cent Coin A Quantitative Analysis The U.S. Penny • Mass of penny, if only Cu, worth >$0.01 • Modern penny is ~2.5% copper clad • Knowledge of composition important to prevent hoarding and selling for metal • Gravimetric analysis of recovered copper just one way The Assay mCu Percentage Cu = m ×100 penny mpenny is easy mcopper is a little harder The Process Obtain and weigh 1 penny… …and dissolve it in acid: Zn(s) + HNO3(aq)  Zn(NO3)2(aq) + H2(g) Why↑can’t we use HCl? Unbalanced! ↓ Cu(s) + HNO3(aq)  Cu(NO3)2(aq) + NO2(g) CAUTION! 10 M HNO3 is corrosive to metals. It’s also corrosive to skin and clothes. Any spills on the bench or in the hood must be wiped up immediately Do this reaction in the hood, please The Process Add H2O to reduce acid concentration Reduce the acidity thru reaction with NH3(aq) (ammonium hydroxide) CAUTION! Conc NH3 is corrosive to skin and clothes and has a strongly pungent odor. Any spills on the bench or in the hood must be wiped up immediately Do this reaction in the hood, please The Process Selectively precipitate the Cu… Add NaBH4 in excess 4 Cu2+ + BH4- + 3 H2O  4 Cu(s) + BO33- + 10 H+ Do this reaction in the hood, please Why do we lower the acidity? Promote BH4- reaction with Cu2+ How do we get rid of the excess BH4-? Let it react with the water already present The Process Recover the Cu by vacuum filtration Rinse the filter paper with water Eliminate water with ethanol Dry at 80º for 30 min The Process Dissolve 1 weighed penny Zn(s) + HNO3(aq)  Zn(NO3)2(aq) + H2(g) Cu(s) + HNO3(aq)  Cu(NO3)2(aq) + NO2(g) REVIEW Reduce acidity and precipitate the Cu 4 Cu2+ + BH4- + 3 H2O  4 Cu(s) + BO33- + 10 H+ Recover the Cu by vacuum filtration Dry at 80º, weigh This investigation, Author 1: Introduction and Conclusion 2: Discussion 3: Data/Results and Experimental This investigation, Author A: Introduction, Conclusion, Data/Results B: Discussion and Experimental