Ruth Zalalem (1) - Percent Copper in Brass - Checked on 10_11

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Feb 20, 2024

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Percent Copper in Brass Background: Brass is a generic term for alloys of copper and zinc. In addition to these metals, brass may also contain small amounts of lead, iron, aluminum, and tin. More than 300 different brass alloys are known, with uses ranging from decorative hardware to architectural construction, musical instruments, and electrical switches. The amount of copper in brass affects its color, hardness, ductility, mechanical strength, electrical conductivity, corrosion resistance, etc. Visible spectroscopy provides a simple tool for determining the percent copper in brass. Spectroscopy involves the interaction of electromagnetic radiation and matter. The absorption of electromagnetic radiation results in different types of transitions in a substance, depending on the energy of the radiation. Low energy microwave radiation, for instance, is converted to energy of molecular rotation. Absorption of infrared radiation excites vibrational frequencies associated with covalent bonds in a molecule. Visible and ultraviolet light cause electron transitions between different electron energy levels in a substance. The energy of electromagnetic radiation is quantized, as are the energy levels associated with various transitions, whether rotational, vibrational, or electronic. Furthermore, the energies of these transitions are characteristic of an atom, molecule, or compound. As a result, the absorption spectra of substances generally consist of specific lines or bands that can be used as a type of fingerprint to identify a substance. Transition metal ions have filled or partially filled d orbitals. The presence of water molecules or other ligands surrounding a transition metal ion in solution leads to energy differences among the d orbitals. Depending on the metal ion involved, the energy difference may correspond to different wavelengths and energies of visible light. This property of transition metal ions gives many their characteristic-and beautiful-colors. The concentration of a colored transition metal ion solution can be determined by measuring the color intensity. A visible spectrophotometer is used to measure the absorption of visible light. In general, absorbance is proportional to concentration. The higher the concentration of a transition metal ion solution, the more intense its color will be, and the greater its absorbance. The linear relationship between absorbance ( A ) and concentration ( c ) is expressed in Beer’s Law (A = abc), where b is the path length in cm and a is a proportionality constant. If c is given in units of molarity (M = moles/L), then a is known as the molar absorptivity coefficient, with units M -1 cm -1 . Beer’s Law can be used to determine the “unknown” concentration of a metal ion in solution if its absorbance is measured. The most accurate way to do this by means of a standard graph called a calibration curve. Plotting absorbance versus concentration for a series of standard solutions, gives rise to a straight line that passes through the origin. y = mx + b, where y = absorbance, x = concentration, and b = 0 Using a calibration curve for quantitative analysis evens out fluctuations due to random error and establishes the range of concentration values over which Beer’s law is valid.

Experimental Overview: The purpose of this lab is to analyze the amount of copper in brass using visible spectroscopy. Brass can be dissolved by reacting it with concentrated nitric acid, which oxidizes the possible metal components of the alloy to their most common ions, Cu 2+ , Zn 2+ , and Fe 3+ . Solutions containing copper (II) ions have a distinctive color. As with most colored solutions, there is a relationship between the concentration of the solution and the amount of light that the solution absorbs. You will make a standard solution containing the copper (II) ion. Using that standard solution, you will make some other solutions containing the copper (II) ion that are less concentrated. You will be able to calculate the concentrations of all these solutions. You will measure the absorbance of each of these solutions using a colorimeter. A graph of absorbance versus concentration will then be constructed. This is called a calibration graph . After the calibration graph is complete, you will react a known mass of a sample of brass with nitric acid. Copper (II) ions will be produced in the resulting solution. The solution’s absorbance will be measured, and its concentration will be read from the graph. Finally you will be able to calculate the percent copper in the original brass sample. Pre-Lab: 1. Dissolving brass requires an oxidizing acid such as concentrated nitric acid. Nitrogen dioxide is produced as a by-product in this reaction. Write a balanced chemical equation, ionic equation, and net ionic equation for the reaction of copper metal with concentrated nitric acid to produce copper (II) nitrate, nitrogen dioxide, and water. Cu(s) +4HNO3(aq) →Cu(NO3)2 (aq) + 2NO2( g) + 2H2O(l) Cu + 4H + 4NO3 ⁺ ⁻ → Cu² + 2NO3 + 2NO2 + 2H2O ⁺ ⁻ Cu+ 4H⁺ + 2NO3⁻ → Cu²⁺ + 2NO2 + 2H2O 2. Nitrogen dioxide is a toxic, reddish-brown gas. What safety precautions are needed in this inquiry lab to protect against this hazard, as well as the hazards due to the use of concentrated acid? Gloves, safety goggles, work in a fume hood 3. Copper (II) ions appear blue in aqueous solutions. This is the transmitted color. The wavelengths of light that are NOT absorbed give rise to the perceived or transmitted color of a substance. Based on the principle of complementary colors, which colors or wavelengths of light would you expect to be most strongly absorbed by Cu 2+. ions? orange

4. Spectroscopy measurements may be made in either percent transmittance (%T) or absorbance (A). Based on the mathematical relationship between absorbance and transmittance, A = -log T, explain why calibration curves of absorbance versus concentration may deviate from a straight line when A < 0.1 and when A >1 If it was less than .1 and greater than 1, this would fall out of the 0% and 100% range, so since this is not possible there would not a be straight line because there is no longer a direct proportion. 5. What are alloys? metallic substance composed of two or more elements, as either a compound or a solution . 6. The copper content of typical brass samples can vary widely, but generally there is 75% by mass of copper in brass. Assume that you are using 0.300 g of an alloy that is exactly 75.0% copper by mass. a. Determine the mass and number of moles of copper that will be present in your sample. (0.300 g brass)(0.75)= 0.225 g Cu 0.225 g Cu / 63.55 g Cu = 0.00354 mol Cu b. Determine the number of moles of nitric acid needed to completely react with the number of moles of copper that you just calculated. (0.00354 mol Cu)( 4 mol HNO3 / 1 mol Cu)= 0.00142 mol HNO3 c. Determine the volume of 6.0 M nitric acid that will be needed to completely consume all of the copper in your hypothetical brass sample. Express your answer in milliliters, mL. L= mol/ M ((0.00142 mol HNO3) / ( 6.0 M HNO3 ) ) (1000) = 2.4 mL HNO3 7. The actual volume of nitric acid that you use is likely going to be significantly larger than what you calculated in #6c. Why might an excess of acid be helpful in this determination? Excess acid will allow there to be the most product possible after the limiting reactant is used. 8. If your sample of brass as described in #6 is used to make 10.0 mL of solution, what will be the molar concentration of copper (II) ion in your solution? M= mol / L (0. 00354 mol Cu) / ( 0.01 L) = 0.354 M 9. If your 0.300 g brass sample is used to prepare 10.0 mL of solution, what is the minimum mass percentage of copper that will produce a solution that is at least 0.10 M in Cu 2+ (aq) . (0.01 L)( 0.10 M)= 0.001 mol Cu (0.001 mol Cu)/ (63.55 g Cu)/ 1 mol Cu= 0.063 g Cu ( (0.063 g Cu )(0.300 g brass) ) (100)= 21.18% Cu Safety Precautions: Concentrated nitric acid is severely corrosive, a strong oxidizer, and toxic by ingestion and inhalation. Reactions of nitric acid with metals generate nitrogen dioxide, a toxic, reddish-brown gas. Work with nitric acid only in a fume hood. Copper (II) sulfate solutions are toxic and irritating to skin and body tissue.

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Materials: Electric scale Colorimeter Cuvette Wash bottle Lens tissue 10 tests tubes Test tube rack Beaker Fume hood Nitric acid Watch glass Pipette Glass stirring rod Buret Micropipette Parafilm Nitric acid Cu solution Brass Bullett Flask Brass solution Graphing calculater Procedure - Part 1: Calibration Curve for Cu 2+ Solutions 1. Using the 0.4 M Cu(SO 4 ) 2 solution and distilled water, create a series of dilutions in the mini test tubes. Test Tube Concentration (M) Volume of 0.4 Cu(SO 4 ) 2 (mL) Volume of distilled water (mL) 1 0.05 0.50 3.50 2 0.10 1.00 3.00 3 0.15 1.50 2.50 4 0.20 2.00 2.00 5 0.25 2.50 1.50 6 0.30 3.00 1.00 7 0.35 3.50 0.50

8 0.40 4.00 0.00 2. Set the colorimeter to 470 nm.Place a cuvette that is about ⅔ full of distilled water into the colorimeter. Handle cuvettes at the top so no fingerprints are in the light path. Polish cuvette with a lens tissue. Press CAL. This calibrates the colorimeter to a wavelength of 470 nm. Once you have calibrated your colorimeter, you do NOT have to do it again. 3. Pour out the distilled water from the cuvette. 4. Fill the cuvette ⅔ full with solution from test tube #1. Wipe the sides with lens tissue. Place it in the colorimeter and record the absorbance value. 5. Pour out the cuvette back into test tube #1 and then rinse thoroughly with distilled water over the sink. 6. Repeat steps 4 and 5 from the remaining 7 test tubes. Procedure - Part 2: Percent Copper in Brass 1. Mass the brass bullet casing and record in the data table. 4.0 g 2. Place the brass bullet casing in a beaker and cover completely with 15.8 M nitric acid. This step should be done in the fume hood!! Place a watch glass over the beaker. 3. Pace the beaker on a hot plate at a low setting for 10-20 minutes until it has completely dissolved. 4. Once the beaker has completely cooled, add 30 mL of distilled water, stir with a stirring rod and remove from the hood. 5. Transfer the solution in the beaker to a flask and add until the total volume is 100 mL. Use a wash bottle to rinse the beaker of all its contents. 6. Cover the flask with parafilm and gently swirl the solution. 7. Repeat steps 2 and 3 from Part 1. 8. Fill a cuvette ⅔ full with the brass solution and place it in the colorimeter. Record the absorbance value. Data - Part 1: Concentration (M) Absorbance 0.05 0.042 0.10 0.079 0.15 0.085 0.20 0.130 0.25 0.135 0.30 0.171 0.35 0.181 0.40 0.202

Data - Part 2: Absorbance of Brass Solution: 0.201 Analysis: 1. Create a graph that compares Absorbance (A) vs. Cu 2+ Concentration (M). Construct your graph on graph paper and attach a picture of your graph below or you can use Excel to create a graph and import it below. Be sure to include the best-fit straight line for the data and determine the slope. y= .4507x + .02671 X axis- Concentration (M) 2. Using the calibration graph, determine the concentration of your brass sample solution. 0.201= 0.4507x + 0.02671 x= 0.3867 concentration = 0.387 M 3. Using half-reactions, balance the equation in an acidic solution: NO 3 - + Cu → NO + Cu 2+ 3e- + 4H+ + NO3- → NO + 2H2O reduction Cu → Cu2+ + 2e- oxidation 8H+ + 2NO3- + 3Cu → 2NO + 4H2O + 3Cu2+ 4. Using the concentration determined in #2 and stoichiometry, calculate the number of grams of copper in the brass sample. Remember there was 100 mL of the solution. 0.387 M/ 0.1 L = 0.0387 mol Cu (0.0387 mol Cu)(63.5 mol Cu)= 2 g Cu 5. Calculate the percentage copper in your brass sample. ((2 g Cu)/ ( 4.0 g brass)) (100)= 50 % Cu Conclusion: 1. Assume the calculation for the amount of nitric acid was too small. How could this impact the calculated percentage? If there was less amount of nitric acid, the calculated percentage would be lower. This is because the nitric acid is used to dissolve the bullet, by Y axis- Absorbance (A)

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removing the Cu from the bullet. This happens because the copper replaces the hydrogen in nitric acid. If the nitric acid was too low, all of the copper would not be removed, therefore the percentage would be less. 2. Identify two sources of error in the experiment and explain how this could have affected the percent of copper in the brass. - Did not thoroughly rinse the beaker of all its contents, therefore some Copper was still left in the beaker causing the percent of copper to be lower - Not leaving the solution in the bunsen burner an enough amount of time, this would cause the bullet to not fully dissolve, causing a lower percent of copper.