[Propitious]

Chemistry experiment: finding the amount of zinc in pennies
 
The method I used to determine the amount of zinc in pennies is to cut up the penny in many tiny pieces (so more zinc will be exposed) and then put the pieces into a flask with 20 mL of 6 M HCl. After the penny dissolved I titrated the dissolved penny/HCl solution with 3 M NaOH solution. I used phenothilene as the indicator.
 
Side info:
I made the NaOH solution by adding 140 grams of NaOH into 1 liter of distilled water. The teacher said 20 grams into 1 liter will give me a .5 M NaOH solution so I figured adding 6 times the much would give me 3 M NaOH solution. I was going to make 6 M NaOH solution so it would be a 1:1 ratio with the 6 M HCl but the teacher did not want me to use that much NaOH for some reason.
 
I also figured out that 20 mL of the 6 M HCl solution should be plenty to dissove the amount of zinc in a penny by this equation:
 
.02 liters of HCl x (6 M HCl / 1 L HCl) x (1 mol Zinc / 2 mol HCl) x (65.399 Zinc / 1 mol Zinc) = 3.9234 grams of zinc
so this means (or at least I think it means) that 20 mL of the 6 M HCl should dissolve at least 3.9 grams of zinc
 
 
 
so I followed the method I stated above to find the amount of zinc in pennies
 
the pennies I tested so far are 2001, 2004, 2007 all weighing 2.5 grams
I cut them up and put them in a flask containing 20 mL of 6 M HCl (each penny in a seperate flask obviously)
 
after a day the pennies completely dissolved which I was supprised about at first because I thought the copper would remain
the solution turned slightly aqua green which obviously means the copper dissolved
 
I then titrated with the 3 M NaOH solution (adding the phenothilene as indicator so the solution will turn pink when the reaction is done) I made and the results were: 2001 - 33.6 mL of NaOH solution until the solution turned pink
2004: 30 mL of NaOH
2007: 28.4 of NaOH
 
I'm not sure exactly why the results varied so much but I'm not too worried about that at the moment. I figure it just has to something with my crappy titration skills.
 
Plugging the results into the formula:
 

.033 Liters of NaOH x (3m NaOH / 1 Liter NaOH) x (1 mol ZnCl2 / 2 mol NaOH) x (136.3 ZnCl2 / 1 mol Zn Cl2) = 6.75 g ZnCl2
6.75 g ZnCl2 (1 mol ZnCl2 / 136.3 g ZnCl2) = .0495 mol
 
shouldn't the number be closer to around 2.4 something because that should be the amount of zinc in a post 1983 penny (95% zinc)?

 
There is something obviously wrong with the number. It should be closer to 2.4. Please help me figure out why I am getting such weird results.
Are my numbers wrong? Is the formula I'm using incorrect? Please, I am stuck and in dire need of a solution to my problem.
 
Thanks for your time and help

[Borek]

Please start with reaction equations. We can try to guess what and why you think you did, but it will be much better for you and us if you will explain chemistry behind the experiment first.

[Propitious]

O right. Of course.

The reaction equation for the formula used to determine the amount of zinc the HCl could react with is

Zn(s) + 2HCl (aq) --> ZnCl2(aq) + H2(g)

The reaction equation for the titration with the NaOH is

ZnCl2 + 2NaOH --> Zn(OH)2 + 2NaCl



Also, the reasoning behind the titration with the NaOH is that the acid will get nutralized. Once I determine the amount of NaOH solution I need to use to nuturalize the penny with HCl, that number will show me how much zinc is in the penny with the formula I used up top.

Thanks

[Arkcon]

Also, the reasoning behind the titration with the NaOH is that the acid will get nutralized. Once I determine the amount of NaOH solution I need to use to nuturalize the penny with HCl, that number will show me how much zinc is in the penny with the formula I used up top.

Thanks

Right.  We'd like that reaction as well.  Also, unless you can separate the HCl -- NaOH reaction, and the ZnCl2 -- NaOH reaction, you won't be able to use NaOH to quantify zinc.  You also noticed the blue color of copper in solution, you will have to account for that, somehow.

Your experiment is a pretty tough one to crack, stuff seems to be all over the place.  Correct chemical equations are a start, but we have a long road ahead.

[sjb]

Is it about here where I pop up and remind people of the importance of accurate names of compounds and the like? 

I didn't think pennies had any zinc in them. Most these days are copper plated steel, or bronze. How might that affect things?

http://en.wikipedia.org/w/index.php?title=British_one_penny_coin&oldid=203729481

[Arkcon]

The US penny is still copper plated zinc, thanks to a stubborn lobby of zinc producers.  And lots of people online sometimes forget to think globally. 

[Borek]

To call cent a penny is funny

[Arkcon]

To call cent a penny is funny

Hmm, funny indeed: http://en.wikipedia.org/wiki/Pfennig

Ah, English, what is it?  German, spoken with a French accent.

[Propitious]

Also, the reasoning behind the titration with the NaOH is that the acid will get nutralized. Once I determine the amount of NaOH solution I need to use to nuturalize the penny with HCl, that number will show me how much zinc is in the penny with the formula I used up top.

Thanks

Right.  We'd like that reaction as well.  Also, unless you can separate the HCl -- NaOH reaction, and the ZnCl2 -- NaOH reaction, you won't be able to use NaOH to quantify zinc.  You also noticed the blue color of copper in solution, you will have to account for that, somehow.

Your experiment is a pretty tough one to crack, stuff seems to be all over the place.  Correct chemical equations are a start, but we have a long road ahead.

well I believe that reaction equation is
ZnCl2 + 2NaOH --> Zn(OH)2 + 2NaCl

If I can find the amount of zinc, with simple math I should be able to find out how much copper is in the penny too.


http://www.usmint.gov/about_the_mint/fun_facts/index.cfm?flash=yes&action=fun_facts2

pennis post 1982 are 97.5% zinc and 2.5% copper.

any help would be great

[Borek]

well I believe that reaction equation is
ZnCl2 + 2NaOH --> Zn(OH)2 + 2NaCl

Think: what happens if there was excess HCl added to dissolve penny?

[Propitious]

well I believe that reaction equation is
ZnCl2 + 2NaOH --> Zn(OH)2 + 2NaCl

Think: what happens if there was excess HCl added to dissolve penny?

Well I can’t imagine anything could go wrong if extra HCl was added. There would just be extra fluid swishing around in the flask. Right??

[Borek]

NaOH + HCl -> ?

[Propitious]

NaOH + HCl -> ?

Do I need that equation? I didn't think that was needed.

Well let me figure that equation out