AP Chemistry · Handsworth Secondary 2026–27 · Unit 3

Lab · Percent Copper in Brass

Dr. Ras Mulinta
Handsworth Secondary
Beer–Lambert Law

Brass is a substitutional alloy of copper and zinc. In this lab you dissolve a measured piece of brass in nitric acid, which oxidizes the copper to its blue Cu²⁺ ion, build a calibration curve from standards of known concentration, and use the Beer–Lambert Law to find the % copper by mass in your sample.

CED 3.12–3.13 (Spectroscopy & Beer–Lambert) · Unit 3.  ~2 hours (one double block) ·  GRADED TEAM formal report: written up and scored on the shared AP Chem lab rubric.

Name:Block:Date:Partner (setup only):

Background CED 3.12–3.13

Nitric acid oxidizes both metals in the alloy. Zinc ions are colourless; copper(II) ions are blue, so the colour of the solution comes from Cu²⁺ alone, the dissolved zinc does not interfere.

Reactions: Your tubes hold 6 M (dilute) HNO₃, so the primary reaction releases NO (colourless), which oxidizes to brown NO₂ the instant it meets air at the hood mouth:
3 Cu(s) + 8 HNO₃(dilute) → 3 Cu(NO₃)₂(aq) + 2 NO(g) + 4 H₂O(l)
3 Zn(s) + 8 HNO₃(dilute) → 3 Zn(NO₃)₂(aq) + 2 NO(g) + 4 H₂O(l)
2 NO(g) + O₂(g) → 2 NO₂(g)  (colourless → brown, in air)
With concentrated HNO₃ the dominant product would instead be brown NO₂ directly (Cu + 4 HNO₃ → Cu(NO₃)₂ + 2 NO₂ + 2 H₂O); we use 6 M, so NO forms first.
Beer–Lambert Law: A = εbcabsorbance is directly proportional to concentration. ε is the molar absorptivity (constant for Cu²⁺ at 650 nm), b is the path length (cuvette = 1.00 cm), c is [Cu²⁺]. A graph of A vs. [Cu²⁺] is a straight line through the origin (A = 0 when c = 0), so the slope = εb. Read your unknown's [Cu²⁺] straight off that line.

Why 650 nm? Cu²⁺ absorbs most strongly in the red/orange (≈650 nm), which is why the solution looks blue. The instrument is set to that wavelength so absorbance tracks [Cu²⁺] most sensitively.

Safety read before you start

Concentrated & 6 M HNO₃, corrosive. Nitric acid causes severe skin and eye burns and stains skin yellow. Mrs. Kathy (lab tech) / Dr. Mulinta handle the concentrated acid in the fume hood; you work only with the pre-measured 6 M HNO₃ tubes already set in the hood.
NO₂ gas, toxic, brown. The dissolving reaction releases nitrogen dioxide. All brass-dissolving happens in the fume hood (Room 3035) only. Never carry an actively-fizzing tube out of the hood.
PPE, every student, the whole period: splash goggles, lab coat or apron, nitrile gloves when handling any acid solution. Tie back hair. No food or drink.
The dissolved solution is still acidic. Handle the diluted blue solution with care and wash your hands after transfers, even if you think none spilled.
Disposal: all blue (Cu²⁺) solutions go in the labelled copper waste container, never down the sink. Spills: alert Dr. Mulinta, neutralize with the sodium bicarbonate kit at the bench.

Materials & Equipment low-gear setup

Most glassware is everyday school chem stock. The only specialized instrument is the colorimeter/spectrophotometer, which the class shares at one station.

Per student: pre-cut brass wire sample · glass stirring rod (to coil the wire) · masking-tape label · one 6 M HNO₃ test tube (pre-set in the hood) · 25 mL graduated cylinder · 10 mL graduated cylinder · 100 mL volumetric flask + cap · medicine dropper · 100 mL beaker (mixing) · 250 mL beaker (waste) · 5 cuvettes · wash bottle of distilled water.
Shared at stations: colorimeter / spectrophotometer set to 650 nm with a distilled-water blank · bottle of 1.00 M Cu(NO₃)₂ stock · analytical balance (± 0.001 g) · copper waste container · sodium-bicarbonate spill kit.

Mrs. Kathy (lab tech) will pre-set: the labelled 6 M HNO₃ test tubes in the Room 3035 fume hood; the 1.00 M Cu(NO₃)₂ stock bottle; the colorimeter calibrated/blanked at 650 nm; the copper waste container and bicarbonate spill kit. Dr. Mulinta does the concentrated-acid pre-fill in the fume hood.

Procedure do Part 2 while Part 1 reacts

Part 1: Dissolve the brass (start this first)

  1. Obtain a pre-cut piece of brass wire. Coil it tightly around a glass stirring rod.
  2. Mass the coiled wire on the analytical balance to ± 0.001 g. Record as mass of brass.
  3. Make a 1 cm × 1 cm masking-tape label with your initials.
  4. Take the coil and label to the Room 3035 fume hood. Label one of the pre-set 6 M HNO₃ test tubes (each holds ~5–8 mL; check it has no brass in it already) and drop your coil in. Place it in the rack and return to the lab.
  5. Now begin Part 2 while the brass dissolves (~30–40 min). Return to the hood every 7–8 min to observe, gently swirl, and check whether all the metal has dissolved.
  6. When fully dissolved, bring the tube back to the lab. Transfer the solution into a clean 25 mL graduated cylinder. Rinse the test tube once with a small (~1–2 mL) squirt of distilled water and add the rinse to the cylinder. (Your tube held only 5–8 mL of 6 M HNO₃, so the transfer plus rinse stays well under 25 mL.)
  7. Add distilled water to bring the volume to exactly 25.0 mL. Pour back into the (rinsed) test tube and back to mix thoroughly. If the combined transfer already exceeds 25.0 mL, you over-rinsed: use minimal rinse volume next time and ask Dr. Mulinta before continuing.
  8. Fill a clean cuvette ¾ full with this solution. Read its absorbance at 650 nm. Record as A(unknown).

Part 2: Build the calibration standards (while Part 1 reacts)

  1. Measure 20.0 mL of 1.00 M Cu(NO₃)₂ in the 25 mL graduated cylinder and transfer to the 100 mL volumetric flask. Add distilled water to the mark; cap and invert to mix. This diluted solution is your stock. Use C₁V₁ = C₂V₂ to find its [Cu²⁺]. Pour it into the 250 mL beaker.
  2. Make five standards (A–E). For each: measure the stock volume below into the rinsed 10 mL graduated cylinder, then add distilled water to the 10.0 mL mark. Pour into the clean, dry 100 mL beaker to mix, then fill a labelled cuvette ¾ full. Rinse + dry the cylinder and beaker between standards.
StdStock (mL)dH₂O (mL)Total (mL)
A2.008.0010.00
B3.007.0010.00
C4.006.0010.00
D6.004.0010.00
E8.002.0010.00
  1. Take all five cuvettes to the colorimeter. Read and record each absorbance at 650 nm. Do not mix up which cuvette is which.

Data Tables fill in lab

Brass sample & unknown

Mass of brass wire (± 0.001 g)__________ g
Final volume of dissolved solution25.0 mL
Absorbance of unknown at 650 nmA = __________
[Cu²⁺] of diluted stock (from C₁V₁=C₂V₂)__________ M

Calibration standards   compute each [Cu²⁺] from V₁C₁ = V₂C₂, V₂ = 10.0 mL, C₁ = stock concentration.

StdStock (mL)[Cu²⁺] (M)Absorbance (650 nm)
A2.00____________________
B3.00____________________
C4.00____________________
D6.00____________________
E8.00____________________

Analysis Questions show all work · units · sig figs

Answer on your own paper / in your typed report. Show every formula and substitution, full credit needs the math, not just the answer.

  1. Diluted stock concentration. You diluted 20.0 mL of 1.00 M Cu(NO₃)₂ to 100.0 mL. Use C₁V₁ = C₂V₂ to find the [Cu²⁺] of your stock solution.
  2. Standard concentrations. Using V₁C₁ = V₂C₂ (V₂ = 10.0 mL), calculate [Cu²⁺] for each standard A–E. Fill these into your data table.
  3. Calibration curve. Plot absorbance (y) vs. [Cu²⁺] (x) for the five standards. Draw the best-fit straight line through the points and through the origin (0, 0). The graph must fill ≥ ¾ of the page, with a title and labelled, unit-bearing axes.
  4. Slope = εb. Determine the slope of your best-fit line (include units). Since b = 1.00 cm, state the molar absorptivity ε of Cu²⁺ at 650 nm.
  5. Unknown concentration. Use your line (A = slope × c) with A(unknown) to solve for [Cu²⁺] in the dissolved brass solution.
  6. Moles of copper. The dissolved solution was exactly 25.0 mL. Find the moles of Cu²⁺ (= moles of Cu) in your sample.
  7. Mass of copper. Convert moles of Cu to grams (M = 63.55 g/mol).
  8. Percent copper. Calculate the percent copper by mass in the brass: (mass Cu ÷ mass brass) × 100%.
  9. Reasoning. Common yellow/cartridge brass is roughly 60–70% copper. Does your result fall in that range? If your line had a non-zero y-intercept, what does that tell you about your blank or technique?
  10. Error analysis. Identify one source of error that would make your % copper read too high and one that would make it read too low. For each, explain the direction of the effect through A = εbc.
AP Chemistry · Unit 3, Percent Copper in Brass by the Beer–Lambert Law · Dr. Ras Mulinta · Handsworth Secondary 2026–27. Procedure adapted from a proven school version; pegged to the College Board AP Chemistry CED (Spectroscopy & Beer–Lambert, topics 3.12–3.13) and the AP recommended lab "Colorimetric/Spectrophotometric Analysis." GRADED TEAM formal report, see the shared AP Chem lab rubric and report format.