Mass of Copper in Penny! HELP!!

Feb 2014
2
0
We preformed an experiment in which we dissolved a 2.2453g penny in 20mL nitric acid. The dissolved penny and acid was transferred to a 100mL flask and diluted with distilled water. 25mL of this solution was pipetted to another 100mL flask, 10mL of ammonia was added, then it was again diluted to the line with distilled water.

Stock solutions were made and and absorbance was measured on a Spec 20. The Beer's law plot was made etc. and the concentration of the unknown copper solution was found to be 0.333 M

I know have to calculate the mass of copper in the penny, the % copper in the penny, the volume copper in the penny, etc. and i dont know how to find the mass!! please help me!!!
 

Unknown008

CHF Hall of Fame
May 2010
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Mauritius
Start by finding the amount of moles of copper in the sample tested for absorbance.
 
Feb 2014
2
0
okay these are the calculations i have so far :

100 mL x ( 1 L / 1000 mL) x 0.0333 M = 3.33x10^-3 moles in dilute solution

( 3.33x10^-3 mole / 25 mL ) x 100 mL = 0.01332 moles in concentrated solution x molar mass = 0.846 grams Cu

This number is way larger than it should be, does that mean I did something wrong in my experiment or am I still making a mistake in my calculations?
 

Unknown008

CHF Hall of Fame
May 2010
1,658
37
Mauritius
Well, first thing to note is that you probably have mistyped the concentration of the copper ions from the results of the absorbance measurement (typing 0.333 M instead of 0.0333 M in your question).

But that said, the calculations are correct as far as I see.

If you ran the complete experiment (all dilutions and the absorbance measurement), then it is probable that some errors were left; e.g. the solutions weren't appropriately mixed before diluting them, or the absorbance experiment wasn't appropriately performed in such a way that errors were introduced.