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How much zinc in pennies

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I am a student and I got an assignment form school to find out the percentage of zinc and copper in penny. I am asked to find the measurements from 2 new pennies and one old so far I measured old penny from 1974 is 3.150g before I did the experiment where I add the HCl to it. The new penny from 2001 is 2.600g before the HCL. And another new penny from 1999 is 2.550g before HCL. After I did the experiment with the adding HCl to the pennies the old penny from 1974 is 3.100g and new penny from 2001 is 6.790g another new penny from 1999 is 0.130g. Now I have to answer how much copper it has in all the pennies and how much zinc it has in old the pennies the I have to have the percentage of zinc and copper from all the pennies . Please please guys help me out because this project is due in 2 days.

Thanks a lot guys

Albin C.deleted
student - Akron, Ohio


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Albin, you're asking us to start from obviously wrong data :-)

The 2001 penny could not possibly weigh two and a half times more after immersion in HCl than it did before immersion. It is a part of experimental procedure and learning to be alert for data that doesn't make sense.

Please explain basically what you think this experiment was about, and in general terms how it's supposed to work, and if any of the data points have double-checked, and I'll be glad to try to help you. But simply giving the percentage zinc/copper answer would be a disservice to you.

Please double check the numbers you posted and we can straighten it out. My feeling is that the data for the 2001 penny is wrong, and the rest are probably right. I also think the general idea is that the HCl will dissolve away the zinc but not the copper, so that the "after" result is the weight of the remaining copper after the zinc is gone. Good luck.

pic of Ted Mooney Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey


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As a head start, here are 3 different copper penny lists I found on the internet. All 3 have different compositions for post-1982 pennies.

(1) From a Lincoln Penny website (same figures as coin "Red Book"):

1909 - 1942,  1944-1962 Composition:  95% copper, 5% tin & zinc Weight:   3.11g Diameter:  19mm     Edge:  plain
1943 - 1943 Composition:  Steel coated with zinc Weight:   2.7g
1962 to 1982 Composition:  95% copper, 5% zinc Weight:   3.11g
1982 to date Composition:  99.2% zinc, 0.8% copper - coated with a pure copper. Weight:   2.5g

(2) US Mint Website:
·The composition was pure copper from 1793 to 1837.

·From 1837 to 1857, the cent was made of bronze (95 percent copper, and five percent tin and zinc).

·From 1857, the cent was 88 percent copper and 12 percent nickel, giving the coin a whitish appearance.

·The cent was again bronze (95 percent copper, and five percent tin and zinc) from 1864 to 1962.

(Note: In 1943, the coin's composition was changed to zinc-coated steel. This change was only for the year 1943 and was due to the critical use of copper for the war effort. However, a limited number of copper pennies were minted that year. You can read more about the rare, collectible 1943 copper penny in "What's So Special about the 1943 Copper Penny.")

·In 1962, the cent's tin content, which was quite small, was removed. That made the metal composition of the cent 95 percent copper and 5 percent zinc.

·The alloy remained 95 percent copper and 5 percent zinc until 1982, when the composition was changed to 97.5 percent zinc and 2.5 percent copper (copper-plated zinc). Cents of both compositions appeared in that year.

(3) TREASURY DEPT. WEBSITE

These coins, which are still being produced today, contain 97.6 percent zinc and 2.4 percent copper. This coin is identical in size and appearance to the predominantly copper cent issued before 1982, but this modification saves the Government an estimated $25 million in metal costs every year.

Considering the sources, I think the first two are both right, even though the numbers are different. The 1st says that the base is an alloy made up 99.2% zinc and .8% copper and the total weight is 2.5 grams. Copper is plated on top of this base alloy, but it doesn't say how much.

This can be figured out from the specs on the Mint site (2). It gives the figures of 97.5% Zn and 2.5% Cu. This must be the breakdown of the entire penny - base plus plating. Assuming this, the total Zn is 2.5 X .975 = 2.4375 gms. The total base weight is 2.4375 divided by .992 = 2.45716 gms. The copper plating weight is therefore 2.5 - 2.45716 = .0428 gms. The total copper weight is .0428 + (2.45716)(.008) = .0625 gms.

The surface area of a penny is about .895 sq in. Therefore, the copper plating weight per unit surface area is .0428/.895 = .0478 gms per sq in. A cubic inch of copper weighs about 146.34 gms (S.G of 8.93 X 2.54 cubed). Therefore, the thickness of the plating is .0478/146.34 = .000327" or, .327 mils.

I hope I got the math right and that the assumptions I made were true. The results sound about right. Note that the figures are production averages. Each individual coin will vary somewhat.

Now you know about what results to look for in your chemistry project. You can approach it in 2 ways: (1) Dissolve the zinc selectively without touching the copper. Then filter, rinse, dry, and weigh the copper residue. (2) Dissolve both Zn and Cu in, say, 10% nitric acid and analyze the solution for either Zn or Cu using some other means.

I would suggest using the 1st method (1). Since zinc is amphoteric, it will dissolve in either acids or bases. Acids you could try are hydrochloric, sulfuric, phosphoric, or acetic. You might even be able to use vinegar, which is 5% acetic (use the white variety so you can see the color of the final solution). For a base, you could try sodium hydroxide.

Ideally, you would dissolve ALL of the zinc and NONE of the copper. If an acid is used, the final solution should not be blue or green, which would indicate that some copper was dissolved. It should be colorless, since the zinc in solution will be colorless. The copper plating will probably come out as an empty shell. The .8% copper in the base alloy will come out as a black powder after dissolving the zinc - you must pick this up when you filter.

Except for the vinegar, start out with a weak solution - maybe 5 to 10%, by volume, for the acids and about 10 gms/liter for the sodium hydroxide. Weigh the coin and, with snips, cut the coin in about 4 pieces - this will expose some zinc edges to the solution and make things go faster. Add them to the room temperature solution. After the fizzing stops or slows down, start heating the solution slowly until it is near boiling (don't boil) and no more bubbles are seen coming off the penny. If any white powder is visible, you may have to dilute with water and heat more to dissolve it. If the white powder persists, COOL COMPLETELY and add a little bit more of the acid or NaOH. (WARNING: never add any of these acids or chemicals, except the vinegar, to a hot or warm solution - it can spit back at you and cause severe burns or blindness. Even when cool, add them slowly with constant stirring, using a full-size face shieldamazoninfo and rubber protective glovesamazoninfo.) Reheat after adding. The end result should be copper foil, black powder, and a water clear solution. Cool, filter, rinse well, dry, weigh, and calculate.

All in all, I would try the vinegar first and the NaOH second.

Work Safe and Smart,

Chris Owen

Chris Owen
Consultant - Houston, Texas


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If pennies aren't made of copper all the time then what else do they have in them? are they made of nickel? the reason I ask this is for a project we're doing in science at Grace Christian school. Our teacher put in an error in all the pennies we used for an experiment. She put the average density of copper as 8.9 and our result was around 10 or 11...how is this possible?

Alexa E. deleted
k-12 student - Morrisville, North Carolina, United States


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You divide the mass of copper by the mass of the penny and multiply it by one hundred and round to your significant figures to get the percent of copper in the penny and whatever is left is the percent of zinc in the penny.

Tim Zimetz
- Chicago, Illinois, United states


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Zinc/Copper in pennies. Why did congress make the materials in pennies change? I've got 6,000 pennies at my house and I can tell a major difference in them, but I do not see the point?

Travisdeleted
- Ironton, Ohio


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This page is being used by students, Travis, so we don't want to blurt out something they are supposed to be thinking through, but here's some hints. Why don't they make pennies out of gold, like they used to make $5 gold pieces many years ago? The second hint is that metals, like everything else, get more expensive all the time.

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Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey

January 31, 2008

The reason the contents were constantly changed is because different metals are like the stock market and the prices of them are constantly changing if the price of a metal went up why would they spend more that a penny to make a penny? so they would change the ratio of materials if one got too expensive. in 1943 solid steel with zinc coating pennies were made to save copper for the war. I know this because thats one of the questions on the project your doing. I'm currently doing the same packet project except we didn't have to drop it in a chemical just water to find the volume. lol

Stacy S.deleted
- Gainsville, Florida, United States


November 24, 2011

We did in chemistry an experiment to get the densities of pennies and my classmates and I got different densities, btw 1-9 cm3 and 7 cm3. How could we have gotten different densities?

Hanna M
- New York, New York, America

November 25, 2011

Hi, Hanna.

It is stating the obvious to say that someone made a mistake in the experiment :-)

Unfortunately, I don't know what procedure you used, and you didn't tell us, so it's a bit hard to know where the mistake happened.

Regards,

pic of Ted Mooney Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey
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