Question

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?

Answer 1

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
http://www.coolmagnetman.com/magconda.htm

Answer 2

Here is a site that gives composition and mass by year for the penny.

http://www.physics.utoledo.edu/~alukasz/Penny_Composition.htm

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.

Answer 3

My ratio above might be misinterpreted. Perhaps I sould have written 97.5%Zn/2.5% Cu.