Homework Help: Percent copper in post-1982 pennies

1. Sep 9, 2013

kevinnn

% copper in post-1982 pennies

1. The problem statement, all variables and given/known data
The total mass of each penny is 2.500g, the density of copper is 8.96g/ml, the density of zinc is 7.14g/ml and the mass of a penny if it was 100% copper would be 3.11g.

2. Relevant equations

3. The attempt at a solution
I am not too sure how to start this one. If someone could give me a boost I would appreciate it.

2. Sep 9, 2013

kevinnn

Sorry not the clearest. I want to know the percent composition by mass of zinc and copper.

3. Sep 9, 2013

SteamKing

Staff Emeritus
You can figure the volume of the penny by using the figures for 100% copper. Once you do that, you can write an algebraic equation for the penny which is composed of a mixture of copper and zinc.

4. Sep 10, 2013

kevinnn

Ok well I got the volume of a penny to be 0.347ml. The only equation I can think to set up is

0.347ml=x(1ml/8.96)+y(1ml/7.14) From this I need a way to solve for x or y.?.?

5. Sep 10, 2013

SteamKing

Staff Emeritus
You don't necessarily want to calculate the volume of the penny; that has already been done. You want to find out what the percentage of copper and zinc are in the new penny which has a mass of 2.500 g.

Whatever part of the new penny which isn't copper must be zinc. So, if x is the percentage of copper, what must the percentage of zinc be in the new penny?

6. Sep 10, 2013

Staff: Mentor

There is another equation. x and y are fractions of zinc and copper - what is their sum?

SteamKing points you in the same direction, he just tries to use a single variable.

7. Sep 10, 2013

kevinnn

Well the answer I'm getting can't be right because it is more than 100%. The equations I set up though are

x+y=2.500g ==> x=2.500g-y

0.357ml=(2.500g-y)(1ml/8.96)+y(1ml/7.14)

x represents copper and y represents zinc.

8. Sep 10, 2013

SteamKing

Staff Emeritus
That's because you're not paying attention. If x = percentage of copper, what must be the percentage of zinc in the new penny?

9. Sep 10, 2013

kevinnn

The percentage of zinc would be (100-x). So I should eliminate the variable y and replace it with (100-x). So my equation would be 0.357ml=(x)(1ml/8.96)+(100-x)(1ml/7.14) Right?

10. Sep 10, 2013

D H

Staff Emeritus
That's not right. You might find it easier to work with x as a fractional part (a number between 0 and 1) rather than a percentage. It's trivial to convert that fractional part to a percentage.

11. Sep 10, 2013

kevinnn

So your saying that I should use (1-x) instead of (100-x)? making my equation
0.375ml=(x)(1ml/8.96)+(1-x)(1ml/7.14)

12. Sep 10, 2013

kevinnn

This equation still does not work. I get a negative number and something greater than 100%. What am I doing wrong?

13. Sep 10, 2013

D H

Staff Emeritus
Don't drop units! If you had kept the units intact you would have seen that this is the wrong equation. That 8.96 and 7.14: Those are densities. Both terms on the right hand side have units of volume/density, or volume2/mass. The left hand side has units of volume. Your equation has inconsistent units.

Hint: You haven't used the fact that the penny's mass is 2.5 grams.

14. Sep 10, 2013

SteamKing

Staff Emeritus
Been trying to tell OP that since Post #5.

15. Sep 10, 2013

kevinnn

The reason my units are just ml on the right hand side is because (x gCu)*(1ml/8.96 gCu) =x(1ml/8.96). The same is true for the other expression.

16. Sep 10, 2013

D H

Staff Emeritus
x has to be unitless.

17. Sep 10, 2013

kevinnn

Well how do I get x unitless? I felt bad about not getting this problem but when I got to class turned out no one else I could find got it either. Are my equations even close to correct?

18. Sep 11, 2013

SteamKing

Staff Emeritus
No. We've been trying to tell you that, but you haven't taken heed of our suggestions.

Forget the volume of the penny. That has already been calculated. It is the same for both the old penny and the new penny.

What you must find out is how much copper and how much zinc it takes to make a penny which has a mass of 2.500 grams.

You can use the mass of the new penny to figure out what the average density of the penny must be, given its mass of 2.500 grams. You also are given the densities of copper and zinc.

Once you know the average density of the new penny, you can write an algebraic equation using the densities of Cu and Zn to find out the proportions of the mixture which gives this average density.

19. Sep 11, 2013

kevinnn

What you must find out is how much copper and how much zinc it takes to make a penny which has a mass of 2.500 grams.
You can use the mass of the new penny to figure out what the average density of the penny must be, given its mass of 2.500 grams. You also are given the densities of copper and zinc.

I calculated the average density to be 2.80g/ml.

Once you know the average density of the new penny, you can write an algebraic equation using the densities of Cu and Zn to find out the proportions of the mixture which gives this average density.

An average density can be useful? That will not yield an inaccurate calculation?

Last edited: Sep 11, 2013
20. Sep 11, 2013

kevinnn

1234 ignore this

Last edited: Sep 11, 2013
21. Sep 11, 2013

SteamKing

Staff Emeritus
The mass of the penny is 2.500 g. You already know the volume of the penny is 0.347 ml. How in the world does that make the average density of the coin 2.80 g/ml?

'That will not yield an inaccurate calculation?' = That will yield an accurate calculation.
Watch using double negatives in a sentence.

Clearly, you are having problems doing simple calculations and it doesn't seem that you understand the concept of density. Always check your work before presenting it, and study harder.

22. Sep 11, 2013

SteamKing

Staff Emeritus
Look at it this way:
if the penny was 100% copper, its density would be 8.96 g/ml
if the penny was 100% zinc, its density would be 7.14 g/ml

Your average density for the new penny must lie between these two figures. It can't be less than 7.14 g/ml and it can't be greater than 8.96 g/ml.

This is the type of problem solved by Archimedes more than 2000 years ago.

23. Sep 11, 2013

kevinnn

That was a typo. I was looking at the wrong calculation in my calculator. I know how to find just and average density, it's 8.05g/ml.

24. Sep 11, 2013

SteamKing

Staff Emeritus
The 8.05 g/ml would be the average density if the penny was 50% copper and 50% zinc.

But you have a penny which has a mass of 2.500 g and a volume of 0.347 ml. The density of the penny will be derived from these two pieces of data.