|
|
|
|
||||||||||||||||||||||||||||||||||||||||||||||||
|
|
Determination of the Percent
Copper in Brass |
|
||||||||||||||||||||||||||||||||||||||||||||||||
|
|
Introduction An important aspect of chemistry is determining
what components are present in a substance.
This is not only important for more publicized things like drug
testing but more mundane things like quality control in industrial parts made
of certain metals. The composition of
metals often plays are part in their strength and other characteristics. In this experiment we are going
to determine the percent copper present in a brass screw. We are going to do this
using two different methods. In the first method we will use density and in
the second we will use a colorimetric assay for copper. Brass is an alloy of copper and zinc and is
usually 65% to 75% copper with the remainder as zinc or other metals. You are already familiar with
using density as a means of evaluating what a substance is. In this experiment you will use density to
determine the percent composition of a brass screw by making the assumption
that the screw is composed only of copper and zinc. In the second part of this
experiment you will dissolve as sample of the screw in nitric acid and then
make up the solution to a known volume.
Copper metal, when it is dissolved by an acid like nitric acid, will
produce a blue colored solution. The
darker or more intense the blue color in solution, the greater the amount of
copper. By using an instrument called
a spectrophotometer, we can measure quantitatively how much light is absorbed
by the solution and thus infer how much copper is present in the solution. We
do this in part by comparing our sample or unknown to a series of solutions
of known concentrations of copper and determining how much of a specific
wavelength of light these solutions absorb. This is called constructing a
standard curve. We then use the
standard curve to estimate the amount of copper in our sample. |
|
||||||||||||||||||||||||||||||||||||||||||||||||
|
|
Materials
|
|
||||||||||||||||||||||||||||||||||||||||||||||||
|
|
Procedure 1. Obtain a screw. Put on
your goggles! Cut off about a 200 mg. Weigh to the nearest mg and record mass. 2. Weigh out approximately
the same amount of copper metal and record its mass to the nearest mg. 3. Place the brass screw in
one 50 mL beaker and the copper metal in another 50
mL beaker. Add
15.0 mL of 6.0 M nitric acid to each beaker. (Do this in the hood!) Allow the sample to dissolve
overnight. 4. (Day 2) Transfer the
dissolved solutions to respective 10 or a 25 mL
volumetric flask. Rinse the beaker with a small amount of distilled water and
transfer the rinse solution as well.
Adjust the volume to 10 mL or 25 mL depending on flask. 5. Clean and dry the
beakers. Transfer the solutions back
to the beakers and clean and dry the volumetric flasks for the next class to
use. 6. Prepare your standard
curve using the copper metal solution, the 5.0 mL
pipette & pipette pump, the test tubes & the test tube rack. Make three unknown determinations. |
|
||||||||||||||||||||||||||||||||||||||||||||||||
|
|
|
|
||||||||||||||||||||||||||||||||||||||||||||||||
|
|
7. Measure the absorbance
of the solution in each tube at 650 nm of light. Measure the absorbance of the unknown.
Record your results in your lab notebook. 8. Prepare a graph of the
standard curve results with concentration of copper on the x-axis and the
absorbance on the y-axis. Use Excel to
determine a line of best fit and an equation for the line. Calculate the
amount of copper and the percent copper in the brass sample that you
dissolved. 9. Using the buoyancy
method that we used for the density determination of an unknown metal,
determine the density of the brass screws.
Use the density information to calculate the percent by mass zinc and
copper. 10. In your discussion
compare the two methods and discuss which you think is more accurate and
why. Also, be sure to discuss
potential sources of error. |
|
||||||||||||||||||||||||||||||||||||||||||||||||
|
|
Questions 1.
What would be the density of an
alloy of 65% copper and 35% tin?. 2. Why
was it necessary to clean and dry the beakers before transferring the
material in the volumetric flaks back into the beakers in the above procedure? 3. Why
is brass used in screws, nuts and other fasteners instead of pure copper? 4. Why
do we not have to take multiple measurements at each volume in making the
standard curve? 5. What
would be the density of an alloy of 30% tin, 30% lead and 40% silver? |
|
||||||||||||||||||||||||||||||||||||||||||||||||
|
|
|
|
||||||||||||||||||||||||||||||||||||||||||||||||
|
|
|
|
||||||||||||||||||||||||||||||||||||||||||||||||
|
|
|
|
||||||||||||||||||||||||||||||||||||||||||||||||
|
|
|
|