AP Chemistry Lab #1: Determination of the mass percent of copper in brass
Introduction:
Brass is an alloy (mixture) containing copper and zinc. When copper reacts with
concentrated nitric acid, it reacts according to the following (net-ionic) equation:
3 Cu(s) + 2 NO3-(aq) + 8 H+(aq)  3 Cu2+(aq) + 2 NO(g) + 4 H2O(l)
When the nitrogen monoxide gas that’s produced is exposed to the oxygen in the room, it
will react to form NO2, a brown gas that is extremely dangerous to inhale. Since the
copper (II) ions that are produced are blue, it is ideal to study the concentration/molarity
of copper (II) ions, [Cu2+], using spectrophotometry. The zinc component of brass, on
the other hand, will react with nitric acid the same way as copper does to give Zn2+, but
since this ion is colorless, it cannot be detected by spectrophotometry.
Purpose:
Your goal in this experiment is to figure out the mass percent of copper in a sample of
brass (or some other copper-zinc mixture).
Pre-lab:
1. Calculate how much Cu(NO3)23H2O(s) is necessary to make 25.00 mL of 0.400
M aqueous solution. Describe the procedure to make this solution assuming that
you have a 25 mL volumetric flask available. How much solute would you need
if you instead made 50.00 mL of 0.400 M solution (in a 50 mL volumetric flask)?
2. Read through the procedure below (in particular steps 6-9). What will the
molarities be of the 5 solutions that you have created?
Procedure:
1. Obtain a sample of solid brass with mass 1-2 grams (probably two brass screws or
3 small brass spheres). Record its mass.
2. In the fume hood, add approximately 7-8 mL of concentrated nitric acid (15.8 M
HNO3) to a (labeled) 50 or 100 mL beaker containing the brass sample. Since this
nitric acid is extremely dangerous, do not measure this volume with a graduated
cylinder; instead use a plastic pipet. You will notice that plastic pipets have
volume markings up to 2 mL. You should need to add about 4 squirts from a
pipet.
3. Allow the reaction to run overnight in the fume hood with the window pulled
down. Recall that NO2 gas will be produced, so you must not remove the beaker
from the fume hood while the substances are visibly reacting.
4. Once the reaction is complete, add approximately 50 mL of distilled water to the
beaker in the hood.
5. Transfer this solution to a 100 mL volumetric flask. Dilute to 100.00 mL, cap it,
and invert it at least 10 times to mix thoroughly. The purpose of diluting to 100
mL is to ensure that the color will not be too intense to get good data.
6. Prepare 25.00 mL of 0.400 M Cu(NO3)2(aq) starting from Cu(NO3)23H2O(s).
Weigh your solid in a beaker and dissolve in a little distilled water before you
transfer to the volumetric flask. Consult your prelab calculations.
 Alternatively your group and another group may be asked to prepare 50
mL of solution together to share.
 Transfer your prepared solution (or share of solution) to a small beaker for
easier pipetting.
7. Use a volumetric pipet and bulb to transfer 10.00 mL of distilled water to each of
5 large test tubes.
8. Use a pipet bulb to push any remaining liquid out of the pipet (into the sink)
9. Use the volumetric pipet again to transfer 10.00 mL of your 0.400 M copper
nitrate solution to one of the test tubes. Mix with a stirring rod. Label this test
tube with its concentration.
10. Repeat steps 8-9 three more times, each time diluting the previous solution.
11. You should now have 5 test tubes and 1 volumetric flask of blue solution (the 5
you just prepared and the brass solution). Pour into 6 cuvettes (fill ¾ full) and
measure the absorbance for each in the spectrophotometer (at 630 nm). Don’t
forget to calibrate the spectrophotometer before you take your measurements.
12. Clean up.
 Your solutions should all be poured into the aqueous waste container. Do
not pour down the drain.
 Every beaker, flask, graduated cylinder, pipet, and cuvette used must be
cleaned and rinsed with distilled water. Plastic pipets are disposable.
 Paper towels must be thrown away.
 Cuvettes should be returned to their box and left on bench top.
Volumetric flasks should also be left on bench tops.
 Everything else should be returned to your cabinet (including wash
bottles).
Calculations and Questions:
1. Prepare a graph of absorbance vs. concentration for your 5 known concentration
data points. Determine the equation for the line of best fit.
2. Calculate the molarity of Cu2+ in your unknown brass solution.
3. Calculate the % of copper in brass.
4. You may have noticed that the Cu(NO3)23H2O (s) absorbs water from the air as
you’re weighing it out (it’s deliquescent). Based on this source of error, will you
calculate your % copper to be higher or lower than the actual %? Explain
thoroughly.
5. You probably had to use wet volumetric pipets in this lab. Based on this source
of error, will you calculate your % copper to be higher or lower than the
actual %? Explain thoroughly.
6. You probably also had to use wet volumetric flasks in this lab. What effect will
this have on your calculated % of copper? Explain thoroughly.
7. A student reacted 50.00 mL of Cu(NO3)2 (aq) with absorbance = 0.685 with
excess zinc. What mass of solid copper will form if the reaction below occurs?
Zn (s) + Cu2+ (aq)  Cu (s) + Zn2+ (aq)