posted by Beth on .
My physical science class is doing an experiment on the mass of pennies. I hypothesized that newer pennies would have less mass because cheaper materials are being used. I found that before 1982, pennies were made from 95% copper/5% zinc, and after 1982 from 99.2% zinc/0.8% copper. The mass of pre-1982 pennies is around 3.3 gr, and after 1982 about 2.5 gr. This would seem to support the hypothesis that zinc has less mass than copper, but I can't find any information about mass on the different metals. All I can find is atomic weight, and I don't even know if this relates to mass. Would someone be able to give me information about the mass of metals or know of a website that would have this information?
What you want is the densities (mass per unit volume) of copper and zinc. Metal densites are readily available in chem and physics handbooks and online.
For example, try
Here is a site that gives composition and mass by year for the penny.
You can find the density of each at www.webelements.com and click on copper (#29) and zinc (#30). The density of Zn is a little lower than that of copper and you can see that the penny changed to a lower mass with the new 97.5/2.5 Cu/Zn blend. The atomic mass of zinc is slightly higher than that of copper which means that a mole of Cu atoms weighs slightly less than a mole of Zn atoms.
My ratio above might be misinterpreted. Perhaps I sould have written 97.5%Zn/2.5% Cu.