CHM144 Lab 8 Lab Report FINAL - Finding the Amount of...

Finding the Amount of Copper in Pennies
Samantha Stretar
Partners: Alexandra Swartz and Brenton Sullivan
Michelle Veite, Professor Yasmin Jessa, CHM144D, 11/13/14
Introduction
Strong oxidizing agents are needed to oxidize copper. Nitric acid is an example of a
strong oxidizing agent that has the ability to oxidize less active metals including Cu. The
balanced equation of the oxidization reaction between Cu and HNO
3
is:
Cu ( s ) + 4HNO
3
( aq ) + 2H
2
O ( l ) Cu(H
2
O)
4
2+ ( aq ) + 2NO
2
( g ) + 2NO
3
- ( aq )
Copper is present in pennies along with zinc. When Zn is oxidized by HNO
3
the
following reaction occurs:
Zn ( s ) + 4HNO
3
( aq ) + 2H
2
O ( l )  Zn(H
2
O)
4
2+
( aq ) + 2NO
2
( g ) + 2NO
3
-
( aq )
The oxidation reactions of Cu and Zn produce complex ions of Cu(H
2
O)
4
2+
and
Zn(H
2
O)
4
2+ . The central metal ions, Cu 2+ and Zn 2+ , are covalently bonded to the neutral
molecular ligands H
2
O. These H
2
O ligands can be displaced by ammonia to create two new
complex ions:
Cu(H
2
O)
4
2+ ( aq ) + 4NH
3
(aq)  Cu(NH
3
)
4
2+ (aq) + 4H
2
O (l)
Zn(H
2
O)
4
2+
( aq ) + 4NH
3
(aq)  Zn(NH
3
)
4
2+
(aq) + 4H
2
O (l)
Formation of the new Cu(NH
3
)
4
2+
ion creates a deep blue color because the ion absorbs
red light. The blue color allows for the determination of the concentration of Cu without conflict
of the concentration of Zn. Therefore, aqueous NH
3
is an effective additive to determine the
concentration of Cu in a penny.
The concentration of a compound in a solution is directly proportional to the amount of
light energy absorbed by that compound in the solution at a particular wavelength. A UV-visible
spectrometer is able to measure the amount of light energy absorbed at a particular wavelength
for the Cu(NH
3
)
4
2+ ion.
The main objective of the experiment was to determine the amount of copper present in a
penny. Another objective was to determine if there was a relationship between the Cu content
and the age and condition of the coin.
1

The hypothesis tested was that the percentage of copper present in a penny is around
2.5% by mass based off of Congressional mandate. Various factors could affect the percentage
of copper present in a penny such as the age and condition of the penny sample. Most of the
copper in a penny is present on the outer surface, so wear and tear over multiple years could lead
to erosion on the surface and therefore a decrease in the % of Cu present in a penny.
Methods
I. Preparation of the Standard Solutions and Penny Samples
Forty milliliters of 5.00 g/L stock Cu 2+ solution was measured using a clean beaker. Six, 50-
mL volumetric flasks were obtained and cleaned. The stock Cu 2+ solution, in volumes of 0.00
mL, 2.00 mL, 4.00 mL, 6.00 mL, 8.00 mL, and 10.00 mL, was pipetted into the separate
volumetric flasks.