Percentage of Copper in Brass
Lab Handouts
Student Lab Handout: Document contains basic lab procedure and purpose. Standard for student use.
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Annotated Lab Handout: Lab handout above with basic procedure steps in addition to notes/ annotations that explains the use/meaning behind each step of the procedure. In addition, helpful notes that could be useful to the teacher/ instructor while conducting the lab.
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Data Table: Printable data table to record collected data from the lab
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Chemical/ Material Preparation Notes
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Copper(II) sulfate hydrate
Deionized water
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Balance (analytical)
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Bunsen burner
Crucible with lid
Desiccator (optional)
Crucible Tongs
Ring stand
Clay Triangle
Iron ring
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Brass Dissolution Solution:
Dispose of waste containing Cu2+ ions as hazardous waste in accordance with local regulations. Label containers as "Copper-Containing Waste."Nitric Acid Waste:
Neutralize excess nitric acid with sodium bicarbonate (NaHCO₃) under supervision and dispose of in a designated acidic waste container.General Waste:
Rinse glassware with deionized water and collect all solutions for proper disposal.
Optional Procedure Modifications
The following are possible other hydrates that can be used in this experiment:
BaCl22H2O
MgSO4 7H2O
Some procedures of similar investigations may call for the crucible to be heated before the beginning of the lab and/or use of a desiccator. This is not needed and is usually used when you are looking to be as precise as possible. You can still get good results by skipping the aforementioned steps.
Some procedures may not call for a lid. The purpose of the lid is to avoid splattering of the solid. Regardless, when a lid is used, it is important that it is NOT completely covering the top. There needs to be an opening to allow the water to escape. (That is the purpose of heating the crucible!)
Pre-Prepared Calibration Curve:
Provide a pre-determined calibration curve to save time and focus on sample analysis.
Alternative Brass Sample Size:
Use a smaller brass sample if the spectrophotometer has a low concentration detection limit.
Visual Indicators for Students:
Use an alternative method (e.g., color matching with a known standard) as a backup if spectrophotometer access is limited.
Online Simulation:
Use spectrophotometry simulation software for students to practice data collection and analysis remotely.
Vocabulary/ Important Terms
Spectrophotometry: A method to measure the amount of light absorbed by a solution at a specific wavelength.
Beer's Law: The relationship between absorbance (A), molar absorptivity (ε), path length (b), and concentration (c): A = εbc
Calibration Curve: A graph of absorbance vs. concentration used to determine the concentration of an unknown solution.
Copper(II) Ion (Cu²⁺): The copper ion measured in the solution.
Nitric Acid (HNO₃): A strong acid used to dissolve the brass sample.
Absorbance (A): A measure of how much light is absorbed by a solution.
Molar Absorptivity (ε): A constant that indicates how strongly a substance absorbs light at a specific wavelength.
Dilution: The process of reducing the concentration of a solution by adding more solvent.
Pre-Lab Question Bank (with answers)
What is the purpose of using a calibration curve in this experiment?
Answer: The calibration curve allows us to relate the absorbance of a solution to the concentration of Cu²⁺ ions. It provides a method to determine the concentration of Cu²⁺ in the unknown brass sample.
Why do we use concentrated nitric acid (HNO₃) to dissolve the brass sample?
Answer: Concentrated nitric acid oxidizes copper and zinc in the brass, dissolving the sample and producing Cu²⁺ ions for analysis.
What safety precautions should you take when working with concentrated nitric acid?
Answer: Wear safety goggles, gloves, and a lab coat. Perform the reaction in a well-ventilated area or fume hood to avoid exposure to NO₂ gas, a toxic byproduct.
What is the significance of Beer’s Law in this lab?
Answer: Beer’s Law describes the linear relationship between absorbance and concentration, which is used to determine the Cu²⁺ concentration in the unknown solution.
What is the role of the spectrophotometer in this experiment?
Answer: The spectrophotometer measures the absorbance of light by the Cu²⁺ solution at a specific wavelength, providing the data needed for concentration analysis.
Sample Data
| Parameter | Value |
|---|---|
| Absorbance of brass solution | 0.512 |
| Mass of brass sample | 0.850 g |
| Final solution volume | 250.0 mL |
| ([$Cu^{2+}$] (M)) | Absorbance |
|---|---|
| 0.020 | 0.150 |
| 0.040 | 0.298 |
| 0.060 | 0.450 |
| 0.080 | 0.598 |
| 0.100 | 0.752 |
Sample Analysis/ Calculations
Determine the concentration of Cu²⁺ in the unknown solution:
From the calibration curve, an absorbance of 0.512 corresponds to a Cu²⁺ concentration of approximately 0.068 M.
Calculate the mass of Cu in the brass sample:
Moles of Cu²⁺ in the solution:
Moles of Cu²⁺ = Concentration (M) × Volume (L) Moles of Cu²⁺ = 0.068 M × 0.250 L = 0.017 mol
Mass of copper:
Mass of Cu = Moles of Cu²⁺ × Molar Mass of Cu (63.55 g/mol) Mass of Cu = 0.017 mol × 63.55 g/mol = 1.08 g
Calculate the percentage of copper in the brass sample:
% Cu = (Mass of Cu / Mass of Brass Sample) × 100 % Cu = (1.08 g / 0.850 g) × 100 = 63.53%
Discussion Questions
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The linearity of the calibration curve is crucial for accurate concentration determination. The Beer-Lambert Law (A = εbc) states that absorbance (A) is directly proportional to concentration (c) when the other factors (path length (b) and molar absorptivity (ε)) are constant. A linear calibration curve confirms that this relationship holds true within the measured concentration range.
Linear Calibration Curve: If the calibration curve is linear, it means the Beer-Lambert Law is obeyed. This allows for accurate interpolation of unknown concentrations from their measured absorbance values.
Non-Linear Calibration Curve: If the curve is non-linear, it indicates deviations from the Beer-Lambert Law. This can occur at high concentrations due to various factors (e.g., chemical deviations, instrumental deviations, stray light). If the curve is non-linear, using a linear fit will lead to inaccurate concentration estimations. In these cases, a non-linear fit or dilution of the samples to fall within the linear range is necessary.
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A blank solution is essential for calibrating the spectrophotometer because it:
Sets the baseline: The blank, typically the solvent used to dissolve the samples, is used to set the zero absorbance (or 100% transmittance) point. This corrects for any absorbance or scattering of light by the solvent itself or the cuvette.
Compensates for background interference: The blank accounts for any background absorbance from other substances that might be present in the solvent or the cuvette. This ensures that the measured absorbance is solely due to the analyte (copper ions in this case).
Without a proper blank, the absorbance readings would be artificially high, leading to inaccurate concentration determinations.
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The primary assumption is that copper and zinc are the only significant components of the brass sample. This simplifies the analysis because the measured absorbance is attributed solely to the copper ions.
However, brass can contain other trace metals like lead, tin, or iron. If these impurities are present in significant amounts:
Spectral Interference: These impurities might absorb light at or near the same wavelength as copper ions, leading to an overestimation of the copper concentration.
Chemical Interference: The impurities might react with the reagents used in the experiment, affecting the color intensity of the copper complex and thus influencing the absorbance readings.
Therefore, the presence of significant impurities can reduce the accuracy of the copper determination. A more sophisticated analysis method would be required to account for these interferences.
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If the absorbance is too high (typically above 1 absorbance unit), the Beer-Lambert Law no longer holds, and the readings become unreliable. The solution is to dilute the unknown solution.
Quantitative Dilution: Perform a precise dilution by taking a known volume of the unknown solution and adding a known volume of the solvent.
Measure Absorbance of Diluted Solution: Measure the absorbance of the diluted solution. It should now fall within the linear range of the calibration curve.
Calculate Original Concentration: Use the dilution factor to calculate the concentration of the original undiluted solution.
For example, if you dilute 1.00 mL of the unknown to 10.0 mL, the dilution factor is 10. If the diluted solution has an absorbance corresponding to a concentration of 0.050 M, the original concentration would be 0.50 M.
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As mentioned earlier, impurities can cause both spectral and chemical interferences:
Spectral Interference: If impurities absorb light at the same wavelength as the copper complex, the measured absorbance will be higher than it should be. This would lead to an overestimation of the copper concentration and thus a higher calculated percentage of copper in the brass.
Chemical Interference: If impurities react with the reagents or copper ions, it can affect the stoichiometry of the reaction, which will affect the intensity of the color and therefore the absorbance. This could lead to either an overestimation or underestimation of copper, depending on the nature of the interference.
In summary, impurities can significantly impact the accuracy of the copper determination in brass using spectrophotometry. It's crucial to be aware of this potential source of error and, if necessary, use more advanced analytical techniques to account for them.
Possible Sources of Error
Incomplete Dissolution of Brass Sample:
If the sample is not fully dissolved, the Cu²⁺ concentration will be underestimated.
Spectrophotometer Calibration Errors:
Improper calibration of the spectrophotometer may lead to inaccurate absorbance readings.
Dilution Errors:
Errors in preparing the standard solutions or diluting the brass solution could result in incorrect concentrations.
Interference from Other Ions:
If other metal ions in the brass solution absorb light at the same wavelength, the absorbance data may be skewed.
Loss of Solution During Transfer:
Spillage or incomplete transfer of solutions could affect the final results.
Links to Similar Labs
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2019 AP Chemistry Exam FRQ 3:
Spectrophotometric analysis question involving Beer’s Law and calibration curves.
2015 AP Chemistry Exam FRQ 4:
A question involving calculations related to the composition of an alloy.
2012 AP Chemistry Exam FRQ 6:
A problem focusing on the use of experimental data to determine the concentration of a species in solution.
Auto Analysis: Experimental Data Entry & Automated Calculations
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