[Punintended]

Hello,

For my Chemistry lab, we were asked to determine the amount of copper and zinc in coins older than 1983 and newer than 1984.  Since starting in 1983, some U.S. mints started to change the composition of the one-cent coin.  This change was completed by 1984, hence, the amount of copper in a penny was modified during that period (which was actually because of a change in the price of copper)..

Anyway, we developed a method for dtermining the concentration of a copper(II) ion solution.  We put about 50mL of 6 M HNO3 and dropped a post-1983 penny in.  We did the same thing for a pre-1983 penny.  While waiting for the penny to be completely dissolved, 10 mL cuvettes of 0.0 M (blank DI water), 0.4 M, 0.8 M, 0.12 M, 0.16 M and 0.2 M were made from copper using a Mohr pipet and a volumetric flask.  Cuvettes of each of the copper along with cuvettes of both pre- and post-1982 penny solution were made.  A computer program was then used to measure the absorbance and transmittance of each solution.  

Which brings me to my question, I have the values of all the concentrations and their transmittance and absorbance for each.   I have calculated the molarity of the pre-1983 and post-1983 pennies..But I'm not sure how to determine out the mass percentage of copper and zinc in each of the pennies.  Anyone know what to do?

[Qazzian]

Well, assuming you have the known volume of your samples, you can find the total number of moles of the elements in your sample with the molar mass. Now, all you need to do is take that, and divide by the initial mass of the coin. Hopefully you got this before dissolving it.

[Punintended]

Thanks Qazzian! One question, when you say "assuming you have the known volume of your samples" Are you talking about the volume of solution in the cuvettes?

[lemonoman]

Well, the volume of the sample isn't exactly what's needed...it's the mass (as Qazzian also suggested).

I bet what he means is this:  You dropped your penny into the acid, and did some stuff.  Then, we ASSUME you diluted to a certain volume...because the volume you diluted to is going to affect the concentration of iron in your sample.  Knowing that volume, you can just take (Volume)×(Concentration) to get the number of moles (as Qazzian suggest again  ).  Then, you have the mass (number of moles × molar mass) and then you can use that, with the mass of the coin originally, to get the % copper.

P.S. A brand new penny has a certain mass, which is listed on the website of the US Mint (or Royal Canadian Mint...or whatever mint produces coins in your country).  This MIGHT be a good ESTIMATE - but weighing the coin itself (after being cleaned, of course) is best.

P.S.S. While you're on the website, it should ALSO tell you the composition of the coin - to give you a good idea where your number SHOULD be.

[Punintended]

Thanks for clearing that up, lemonoman.  Sorry, I wasn't too coherant the day I did that lab..    I have some more clarifications though if you can bare with me.

You're right, we diluted it to a certain volume.. All of them were 10mL after dilution using a pipet (which changed the concentration) and we had them in seperate labeled Erlenmeyer flasks (0.04 M, 0.08 M, 0.12 M, 0.16 M, etc).  What you're saying is, if I understood correctly, is that now that I know this volume, I can multiply by the concentration (either 0.04 M, 0.08 M, etc) to find the Moles.. Makes sense, this is simply re-arranging the Molarity = Moles / L principle into L * Molarity = Moles.  (As an aside, I should be converting the 10mL to L, right?)  ... Then I convert the Moles I got to grams, and this is the amount of copper in the cuvette.  Then I repeat this for each concentration (0.04, 0.08, etc) of solution... Now I have the total amount of grams of Copper  for each concentration of solution,...  Take each of these, and divide it by the mass of the initial coin, and I have the mass percent of copper.. is this right?

[Punintended]

I think I've got it.. But one thing

When you say to multiple (Concentration) * (Volume) to get moles, WHICH concentration are you talking about?  Are you talking about the concentration(molarity) of the 0.02 M, 0.04 M, etc. cuvettes.. Or the molarity/concentration of each of the pre-1983 and post-1983 pennies?  

[lemonoman]

Well, you used the cuvettes and their known concentrations to find out the exact concentration of copper in the dissolved penny samples.  The 0.2, 0.4, etc were only standards so that you could figure out what the unknown concentration was.

[jdurg]

Also realize that in the USA, the switch over from the copper alloy to the zinc core and copper overlay happened during the year 1982 and was completed by the end of that year.  ALL pennies minted in 1983 have a "pure" zinc core with a thin copper coating.  ALL pennies minted in 1981 are the 'pure copper' variety.  During the year 1982, a mixture of both types of pennies were minted.  So in order to get proper results, you should make sure that you avoid any pennies from the year 1982 as they could be zinc core or copper core pennies.