Lab 8 - Mass Percent of Copper in Brass 2018 PDF

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Unit 6 – Lab #8

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How Can Color Be Used to Determine the Mass Percent of Copper in Brass? adapted from College Board’s “AP Chemistry Guided Inquiry Experiments”, Flinn Scientific, & Vernier

CHALLENGE & CONTEXT FOR THE INVESTIGA INVESTIGATION TION Spectrophotometry is an extremely important tool used in forensic science to determine the detailed chemicals composition of evidence obtained from a crime scene. It can be used to determine the concentration of either single chemicals species in solution or even the concentration of a species within a mixture of species in solution. For example it can be used to determine the mass percent of copper in brass shell casings collected by the crime scene investigator (CSI), and then match the brass composition to a particular manufacturer. The primary objective of the first part of this experiment is to determine the concentration of an unknown copper(II) nitrate solution. You will use a spectrophotometer to measure the concentration of each solution containing copper(II) ions which 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 first make a standard solution containing the copper(II) ion then dilute that standard solution so that you make four additional solutions containing the copper(II) ion that are less concentrated. Additionally, you will know the concentrations of all these diluted solutions. Each standard solution is transferred to a small, cuvette that is placed into the spectrophotometer. The amount of light that penetrates the solution and strikes the photocell is used to compute the absorbance of each solution. When you graph absorbance vs. concentration for the standard solutions, a direct relationship should result. The direct relationship between absorbance and concentration for a solution is known as Beer’s law, and the graph that you create is called a calibration plot or standard curve. The primary objective of the second part of this laboratory exercise is to determine the amount of copper in a sample of brass. You will dissolve the brass and use the calibration curve from Part I to determine the concentration of Cu2+ in your sample which will allow you to calculate the mass of copper in your sample and ultimately the percent of copper in your brass sample.

OBJECTIVES In Part I of this experiment, you will  Prepare and test the absorbance of five standard copper(II) nitrate trihydrate solutions.  Create a standard curve (calibration plot) from the test results of the standard solutions. In Part II of this experiment, you will  Dissolve a brass sample in nitric acid to extract the Cu2+ ion and determine its concentration using your standard curve.  Determine the percent of copper(II) in the original brass sample.

PART I: CREA CREATING TING A CALIBRA CALIBRATION TION PLO PLOT T PART I: PRE-LAB QUE QUESTIONS STIONS 1. What are the name, formula and molar mass of the colored species being measured in this laboratory exercise?

2. How do you propose to determine the volume of your test tubes?

3. The wavelength setting you are told to use in this lab is not chosen arbitrarily. A previous experiment was conducted to determine the best wavelength setting. The experiment used a spectrophotometer and collected absorbance data for a given sample of Cu2+ at 10 nm wavelength increments starting at 450 nm and ending at 850 nm. The graph of the data is shown at right. Explain how this graph is used to determine the wavelength setting for this experiment.

4. A student is given the task of determining the molar concentration for a sample of copper(II) nitrate solution of “unknown” concentration. In order to establish a standard or calibration curve, the student prepares a set of five standard solutions of copper(II) nitrate. The student collects the concentration and absorbance data, and then creates the graph as shown at right.

Calibration Plot for Cu(NO3)2 1.2 1 Absorbance

f(x) = 2.47 x 0.8 0.6

The student then measures the absorbance for the unknown sample and determines it to be 0.669. Determine the concentration of the unknown Cu(NO3)2 solution and explain your method.

0.4 0.2 0 0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

Concentration (M)

MA MATERIALS TERIALS spectrophotometer solid Cu(NO3)2 volumetric pipettes volumetric flasks test tubes test tube rack

PART I PR PROCEDURE OCEDURE 1. Obtain and wear goggles.

balance Kimwipes stirring rod brass sample 15.8M HNO3, nitric acid cuvettes

2. Mass the amount of Cu(NO3)2 • 3 Η2Ο needed to make ≈ 0.400 M Cu(NO3)2 solution within a 100.0 mL volumetric flask. This will be your stock solution. Transfer the solid into the volumetric flask, add water to just below the mark on the flask and mix well. Once the solid has completely dissolved, use a pipette to fill the flask to the mark on the volumetric flask and mix a final time. Record the exact concentration of your solution in the blank provided in #3 below. 3. Determine the volume of each diluted solution you wish to make based upon the size of the 5 test tubes you have available. Your first test tube will simply contain some of your stock solution. Design a data table that includes the concentration and absorbance of each of the 5 standards you chose to make by diluting your __________ M Cu(NO3)2 “stock” solution. Your standards must encompass a range of 0.05 M to 0.40 M. Show your work for all calculations. 4. Using the spectrophotometer, collect and record the absorbance values for each of the five standard solutions taking care to fill the cuvette about ¾ full and wipe off any fingerprints prior to placing it in the spectrophotometer. Create a data table for your values. 5. Use Google Sheets to construct a calibration plot, which is a graph of absorbance vs. concentration. Give your graph a title and axis labels. Be sure to include a line of best-fit and the equation of the line. Print the graph. You will use this calibration curve to determine the amount of copper in your brass sample in Part II. 6. Discard the solutions as directed by your instructor.

PART I: POST POST-LAB -LAB QUESTIONS 1. The molar absorptivity is a measurement of how strongly a chemical species absorbs light at a given wavelength. It is an intrinsic property of the species. Calculate the concentration of a solution having an absorbance equal to 0.600 and a molar absorptivity constant of 1.5 cm−1M −1 with a path length of 1.0 cm. 2. You arrive in the lab to discover that the lab group working at your station did a fine job of cleaning up after themselves. They even rinsed out the cuvettes with deionized water as evidenced by droplets of water clinging to the bottom and sides of the cuvette. What, if anything, should you do prior to measuring the absorbance of your samples? 3. A student fails to wipe the cuvette prior to inserting the cuvette into the spectrophotometer correctly. What effect would a drop of solution left on the outside of the cuvette have on the measured absorbance? 4. A different student fails to wipe the cuvette prior to inserting the cuvette into the spectrophotometer correctly. What effect would a fingerprint left on the outside of the cuvette have on the measured absorbance?

PART II: DETERMINI DETERMINING NG THE PERCE PERCENT NT COPPER IN BRASS Brass is an alloy made of copper and zinc. The proportions of zinc and copper can be varied to create a range of brasses having different properties. It is a substitutional alloy meaning atoms of the two constituents may replace each other within the same crystal structure. By comparison, bronze is principally an alloy of copper and tin. Brass is used for decoration due to its bright gold-like appearance. It is also used in applications to reduce friction in locks, gears, bearings, doorknobs, ammunition casings and valves for plumbing. And you are already aware brass is used extensively for its acoustic properties in musical instruments such as horns and bells. Most importantly, it is also used in zippers! In this part of the experiment, you will dissolve a sample of brass in nitric acid (the oxidizing acid) and use your calibration plot from Part I to determine the amount of copper in your alloy.

PART II: PRE-LAB QUESTI QUESTIONS ONS 1. Write the balanced molecular equation for the reaction taking place with regard to the copper in brass in this laboratory exercise: copper metal reacts with nitric acid to produce aqueous copper(II) nitrate, gaseous nitrogen dioxide, and water. Be sure to include phases! 2. This is an oxidation-reduction reaction. Each of the half-reactions are given below. Write the balanced net ionic equation for the reaction taking place in this laboratory exercise. Oxidation Half-Reaction: Cu(s)  Cu2+(aq) + 2e− Reduction Half-Reaction: 4H+ (aq) + NO3−(aq) + 3 e−  NO(g) + 2H2O(l) (Regents Reminder: Half-reactions can only be combined when the amount of electrons lost equals the amount of electrons gained.) Note: Gaseous nitrogen monoxide, NO, readily reacts with oxygen in the air to produce the reddish-brown gaseous nitrogen dioxide. You do not need to account for this in the net ionic equation, but you maynotice that this is slightly different from the molecular equation you just wrote in #1! 3. A student is tasked with dissolving a brass shell casing with a mass of 3.579 g in 8.0 M nitric acid. Assuming the shell casing is 100% copper, use your net ionic equation from question #2 (above) to calculate the minimum volume of 8.0 M nitric acid the student needs to accomplish this task. Show your work. 4. A student determines the absorbance of their 100.0 mL sample to be 0.669. Calculate the percent copper present in their brass shell casing. (HINT: Use Part I: Pre-lab question #4 to help you!) Show your work.

PART II: PR PROCEDURE OCEDURE 1. Obtain and wear goggles. 2. Obtain a sample of brass (either two shell casings or one screw). Measure the mass to the nearest 0.001 gram and record in a data table. 3. Assuming that your brass sample is 100% copper, calculate the minimum volume of concentrated 15.8 M nitric acid needed to completely react with the brass sample. Show your work. Record the minimum volume value in a data table. 4. Place the brass sample in a small beaker. Under the fume hood, have your teacher add approximately 2 mL more than the calculated HNO3 volume so that the acid is in excess. Your teacher will then cover the beaker with a watch glass. For safety reasons, only the teacher should handle the concentrated nitric acid!! 5. Once the metal has dissolved completely, your teacher will add 50 mL of distilled water to the beaker, being sure to wash down the sides and bottom of the watch glass. You may then remove the beaker from the fume hood. 6. Transfer the solution to a 100.0 mL volumetric flask. Rinse the beaker 3-4 times with 5 mL of distilled water and add the washings to the flask. Dilute to a final volume of 100.0 mL. 7. Determine the absorbance value of the unknown Cu(NO3)2 solution from your brass sample. Record in a data table. Thoroughly rinse out the cuvette once you are done. 8. To clean up, dump your unknown Cu(NO3)2 solution into the waste container and neutralize with a little bit of baking soda – add until bubbling subsides.

PART II: POST POST-LAB -LAB QUESTIONS AND D DA ATA ANAL ANALY YSIS 1. Using your calibration plot from Part I, determine the concentration of the unknown Cu(NO3)2 solution from your brass sample. Show your work. 2. Using your concentration of 100.0 mL of the Cu(NO3)2 solution from the brass sample, calculate the mass percent of copper within your sample of brass. Show your work. 3. A student accidentally spills some of their copper solution prior to determining its absorbance. What effect does this error have on the calculated percent of copper present in the shell casing?

AP Questions:...