Unit 3 Properties of Substances and Mixtures

Percent Copper in Brass Lab

Why the method works, how the procedure fits together, a full sample calculation, and how each lab error changes your answer.

This common AP Chemistry inquiry lab, sold in kits such as Flinn’s Percent Copper in Brass, finds how much copper is in a piece of brass from the color of a solution. It puts the Beer-Lambert law to work with a calibration curve, and it depends on solution skills from molarity and dilution. Kits and teachers differ, so treat this page as a guide to the reasoning and calculations, and follow your own class procedure and safety instructions.

How the Percent Copper in Brass Lab Works

Brass is an alloy of mainly copper and zinc, commonly about 60 to 70 percent copper by mass. Nitric acid oxidizes both metals to ions:

Cu(s) + 4 H⁺(aq) + 2 NO₃⁻(aq) → Cu²⁺(aq) + 2 NO₂(g) + 2 H₂O(l)
Zn(s) + 4 H⁺(aq) + 2 NO₃⁻(aq) → Zn²⁺(aq) + 2 NO₂(g) + 2 H₂O(l)

The method works because the two ions look different: Cu²⁺(aq) is blue, while Zn²⁺(aq) is colorless and absorbs no visible light. All of the absorbance therefore comes from copper. With the wavelength and path length held constant, absorbance is proportional to [Cu²⁺], so standards of known concentration give a line that converts your brass solution’s absorbance into a concentration. Both reactions are redox reactions, written above as net ionic equations.

The Percent Copper in Brass Procedure Step by Step

Safety: concentrated nitric acid is corrosive and a strong oxidizer, and the reaction releases toxic brown NO₂ gas. Dissolving the brass belongs in a working fume hood, with goggles, gloves, and your teacher’s supervision. Do not attempt it outside a supervised lab.

Prepare the standards. Dilute a Cu(NO₃)₂ stock solution to a set of known concentrations, such as 0.050 to 0.400 M. The dilution calculator finds the volumes.
Choose the wavelength. Scan one standard to see where it absorbs most, then keep that wavelength for every reading.
Blank the instrument. Zero it with a cuvette of the solvent used for the standards.
Measure the standards. Rinse the cuvette with each solution, wipe its clear sides, and record the absorbance. Plot absorbance against concentration and fit a line.
Dissolve a weighed brass sample. Record the mass, then have the sample dissolved in nitric acid in the fume hood. The solution may look green at first because of dissolved NO₂; it turns blue when diluted.
Dilute to a known volume. Transfer the solution to a volumetric flask, rinse the beaker into the flask, and add water to the calibration mark. Mix well.
Measure the brass solution. If its absorbance is above your highest standard, dilute it by a known factor and measure again.

How to Choose the Wavelength for a Copper(II) Solution

A solution shows the color it does not absorb. Cu²⁺(aq) looks blue because it absorbs red and orange light, so a scan from 400 to 700 nm shows absorbance climbing toward the red end, and the true peak lies just beyond the visible range. Pick a wavelength in the red where your instrument gives a strong, stable reading. Measuring where absorbance is highest makes the readings most sensitive to changes in concentration, as explained under λ_max in AP Chemistry spectroscopy. Using a wavelength of blue light, which the solution transmits, would give small absorbances that barely change with concentration.

Sample Calibration Data for Copper(II) Standards

The numbers below are realistic sample values written for this page, not measurements from a published experiment or an answer key. Your slope depends on your instrument and wavelength, so your own data will differ.

[Cu²⁺] (M)	0.050	0.100	0.200	0.300	0.400
Absorbance	0.148	0.286	0.571	0.849	1.135

Fig. 1 · Sample calibration curve with the brass solution marked

A least-squares fit gives A = 2.82[Cu²⁺] + 0.006, with R² above 0.999. The slope equals εb, here 2.82 M⁻¹ for a 1.00 cm cuvette, and the small intercept is close to the zero that A = εbc predicts.

How to Calculate the Percent Copper From Absorbance

A 1.502 g piece of brass is dissolved and diluted to 100.0 mL. The solution has an absorbance of 0.437.

Concentration from the line: [Cu²⁺] = (0.437 − 0.006) ÷ 2.82 = 0.153 M.
Moles of copper: (0.153 mol/L)(0.1000 L) = 0.0153 mol Cu.
Mass of copper: (0.0153 mol)(63.55 g/mol) = 0.972 g Cu.
Percent copper: (0.972 g ÷ 1.502 g) × 100 = 64.7% Cu.

The answer falls in the usual range for brass, and the absorbance lies inside the range of the standards, so no dilution was needed. Each Cu²⁺ ion came from one copper atom, which is why moles of ions equal moles of copper; this one-to-one step is ordinary stoichiometry. Report the answer to the significant figures your data supports, and if you diluted the solution, multiply the concentration by the dilution factor before finding moles. If your teacher gives the true composition, the percent error is a natural last step.

How Lab Errors Change the Percent Copper Result

Lab questions ask for a direction and a reason. Trace each error through the same chain: what happens to the measured absorbance or the assumed concentration, then to the calculated moles of copper.

Error	Calculated % Cu	Why
Water drops left in the cuvette before adding the brass solution	Too low	The sample is diluted, so absorbance and calculated [Cu²⁺] are too low.
Brass not completely dissolved	Too low	Some copper never reaches the solution, but the full mass is still in the denominator.
Beaker not rinsed into the volumetric flask	Too low	Copper ions are left behind, so fewer moles are in the flask.
Flask filled past the calibration mark	Too low	The real volume is larger, so [Cu²⁺] is lower, but the calculation still uses 100.0 mL.
Flask stopped short of the calibration mark	Too high	The solution is more concentrated than it should be, but the calculation still uses 100.0 mL.
Fingerprints on the cuvette for the brass reading only	Too high	Smudges block light, so the measured absorbance is too high.
Stock solution more concentrated than its label	Too low	Standards absorb more than their assumed concentrations predict, so the slope is too steep and a given absorbance reads as too little copper.

Using the same wrong wavelength for the standards and the sample does not create a direct error, because the line is built under the same conditions; it only makes the method less sensitive. Changing the wavelength between the standards and the sample does invalidate the line.

Five Brass Lab Post-Lab Questions With Answers

Why can absorbance be used to find copper when the solution also contains zinc ions?
Show answer
Zn²⁺(aq) is colorless and does not absorb visible light, so at the chosen wavelength only Cu²⁺ contributes to the absorbance.
A brass solution has an absorbance of 1.60, above the highest standard (1.135). What should the student do?
Show answer
Dilute it by a known factor so its absorbance lies within the calibration range, measure again, and multiply the concentration from the line by the dilution factor. Extending the line beyond the standards is not justified.
Using the sample line, what is [Cu²⁺] when A = 0.712?
Show answer
(0.712 − 0.006) ÷ 2.82 = 0.250 M.
A 1.300 g brass sample diluted to 100.0 mL has [Cu²⁺] = 0.125 M. What is the percent copper?
Show answer
(0.125 mol/L)(0.1000 L) = 0.0125 mol; × 63.55 g/mol = 0.794 g Cu; (0.794 ÷ 1.300) × 100 = 61.1% Cu.
Would changing to a cuvette with a longer path length after making the calibration curve affect the result?
Show answer
Yes. A = εbc, so a longer path gives a higher absorbance for the same concentration. The brass solution would read as too concentrated, and the calculated percent copper would be too high. Use the same cuvette type throughout.

Solution color and calibration curves also appear on the exam itself; the Unit 3 FRQ practice includes a graded absorbance question. To see how free-response points feed into an overall estimate, try the AP Chem Score Calculator.

Frequently Asked Questions

How do you calculate the percent copper in brass?

Read the Cu2+ concentration of your brass solution from the calibration curve, multiply by the solution volume in liters to get moles of copper, multiply by 63.55 g/mol to get grams of copper, then divide by the mass of the brass sample and multiply by 100.

Why is nitric acid used to dissolve brass?

Copper does not react with hydrogen ions alone, so an acid such as hydrochloric acid will not dissolve it. The nitrate ion in nitric acid is a strong enough oxidizing agent to convert copper metal to Cu2+. The reaction releases toxic NO2 gas, so it must be done in a fume hood.

Why does zinc not interfere with the absorbance reading?

Zinc in the brass also dissolves, forming Zn2+, but Zn2+ in water is colorless and does not absorb visible light. The absorbance at the chosen wavelength therefore comes from Cu2+ alone.

What wavelength should be used for the copper in brass lab?

A wavelength where Cu2+ absorbs strongly, found by scanning a standard. The solution looks blue because it absorbs red and orange light, so absorbance rises toward the red end of the visible range. Use the same wavelength for every standard and for the brass solution.

What is a typical percent copper in brass?

Common brasses are often about 60 to 70 percent copper by mass, with most of the rest zinc, but alloys vary. Compare your result with the value your teacher or the supplier gives rather than assuming one number.

The sample data on this page are written for illustration. This page is not affiliated with or endorsed by College Board or any lab supplier. AP® is a trademark registered by the College Board.