Percent Copper in Brass Background guiding Question

How Can Color Be Used to Determine the Mass Percent of Copper in Brass? Purpose To determine the percent (by mass) of copper in brass using the principles of spectrophotometry and Beer’s Law. ​ Pre-Lab Questions: Designing the Experiment Background on Beer’s Law The absorbance of a given solution in a spectrophotometer is dependent on the depth of the sample (which is equivalent to the width/path length of the cuvette), the concentration of the solution and the molar absorptivity of the solution (a constant for a given solute at a given wavelength). The formula for Beer’s law relates these variables as shown below. Beer-Lambert law: A = ɛbc A = the amount of light absorbed ɛ = the molar absorptivity constant (whose value depends upon the wavelength used and substance tested) b = path length (i.e the length of the cuvette, usually 1 cm) c = molar concentration of the solution Analyzing Beer’s Law: 1.​ Using the formula known as Beer’s law, what is the relationship between absorbance of a solution (A) and…(assume the other variables are held constant) a.​ its concentration (c)? ___________________________________ b.​ the path length (b)? _____________________________________ c.​ molar absorptivity (ɛ)? _________________________________ 2.​ In previous experiments that involved Beer’s Law (i.e Gatorade Lab) you created a standard curve that graphed absorbance (y) vs. concentration (x) producing a line with the equation: ​ y = mx + b Absorbance =m (concentration) + b , where the intercept was 0. Compare the equation for Beer’s Law (A =ɛbc) to the equation above. What part of the equation is equivalent to the slope of the line? Why is the slope constant in the standard curve? 1 Background Question on Brass: 3.​ Draw a particle representation (or insert an image) for Brass (an alloy of copper and zinc). What type of alloy is this? How do you know? How will the malleability of brass be different from copper? Explain your answer. 4.​ Using your drawing, calculate the percent (by mass) of copper in this specific brass sample. *Note: Brass is a solution- a homogeneous mixture of two metals, the ratio of atoms can vary from sample to sample, thus the percent of each element in the sample will also vary. 2 Analyzing & Interpreting Absorption Spectra The graphs below give the absorbance of 4 different solutions from a wavelength of 0 to 800 nm. Use the graphs to answer the following questions. ​ ​ Copper II sulfate ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​​ ​ 5.​ In each solution, what ion is absorbing the light? i.e. the metal ions, the polyatomic anion or both? How do you know? 6.​ Based on your answer to the question (above) and the sample spectra, what wavelength of light should be used to measure the concentration of copper ions in a solution? How did you come up with this answer? 7.​ The colorimeters we have to use for this lab allows you to choose from 4 wavelengths: 430 nm (violet), 470 nm (blue), 565 nm (green) & 635 nm (red). Which color light should be used in this experiment? Why? 3 BACKGROUND ON THE REACTIONS TAKING PLACE In this experiment, you will be extracting the copper in brass through a chemical reaction with nitric acid. You will then analyze the resulting solution to determine the concentration of copper in the solution and then calculate the percent of copper in the brass (by mass). Reaction of Brass with Nitric Acid (Video): Brass is an alloy (a solid solution) of primarily copper and zinc. When reacted with nitric acid (HNO3), metal nitrates are produced that will dissolve in the water as well as poisonous NO2 gas and water. (The reaction will take place in the fume hood.) The reactions are: Cu (s) + 4 HNO3 (aq) → Cu(NO3)2 (aq) + 2 NO2 (g) + 2 H2O(l) Zn (s) + 4 HNO3 (aq) → Zn(NO3)2 (aq) + 2 NO2 (g) + 2 H2O(l) *include these reactions in the introduction section of the lab notebook Since the NO2 is a gas, it will bubble off leaving an aqueous solution containing zinc ions, copper ions and nitrate ions (as shown in the simplified reaction below). ​ Brass (s) + HNO3 (aq) → Zn(NO3)2 (aq) + Cu(NO3)2 (aq) Since the solution produced will contain zinc (II), copper (II) & nitrate ions, you will need to choose the wavelength of light for your spectrophotometer that will be best for detecting the copper ions in the solution. Lab Procedure- How do I prepare the standard solutions and work the lab quest? Digital Lab Notebook Introduction- Purpose & two balanced equations for the reaction of : ​ Cu(s) + HNO3 (aq) → ​ Zn(s) + HNO3 (aq) → ​ ​ ​ Materials/Method- Fill in the blank spaces with the correct answer. 1.​ Measure and record the mass of a sample of brass. 2.​ Prepare standards of copper (II) nitrate from a ___________M stock solution of copper (II) nitrate using serial dilution. The final concentrations of the standards should be ___________________(give all concentrations). 3.​ React the brass sample with_____________ until all the brass has dissolved. Note: This results in two chemical reactions. 4.​ Dilute the resulting brass solution (brass + nitric acid) to __________ mL using a 4 volumetric flask. 5.​ Calibrate the spectrophotometer using a wavelength of ________ nm using _________ as a blank. 6.​ Measure and record the absorbance of the standards.. 7.​ Measure and record the absorbance of the brass solution. Data​ Mass of Brass sample *this will be provided by your teacher Table- Absorbance & Concentration values for standard solutions of copper (II) nitrate ​ Absorbance of the Unknown Brass Sample ​ Graph- *Include an image of this in your digital lab notebook. Absorbance vs. Concentration of standards (copper (II) nitrate) ​ ​ -equation of the line displayed, correlation & RMSE values ​ ​ -axis labeled Calculations- Make sure to label each step! You can write these out, take a photo and add it to your digital lab notebook. Screencast: Sample Brass Lab Calculations 1.​ Calculate the molar concentration of copper ions in the sample (using the equation of the line). 2.​ Calculate the moles of copper ions in the sample (M = mol/L) 3.​ Calculate the mass of copper ions in the sample (mol → g) 4.​ Calculate the percent by mass of copper ions in the brass. (mass Cu/mass Brass) 5.​ Calculate the molar absorptivity of copper (II) ions at 635 nm using the formula A=ɛbC. Note: The path length was 1.00 cm. Check your answer by comparing it to the slope of the line from the graph of Absorbance vs. Concentration! Discussion- Claim, Evidence and Reasoning Post Lab Questions- Add to the end of the lab in your digital lab notebook. 1.​ Using the molar absorptivity for copper II ions (calculated in the lab), calculate the concentration of an unknown copper solution if it has an absorbance of 0.900 at 635 nm with a path length of 1.00 cm. (NOTE: When you have the molar absorptivity value you can calculate concentrations without having to make a standard curve!) ​ Sample Data: This data should be used by students who were absent. Using this data 5 you will need to create a graph of Absorbance vs. Concentration and determine the equation of the line (y = mx +b) DataMass of Brass sample__1.451 g _______ Diluted to a volume of 250 mL Concentration of Cu(NO3)2 standard solutions Absorbance 0.400 M 0.872 0.200 M 0.443 0.100 M 0.226 0.050 M 0.126 0.025 M 0.057 Absorbance of Brass solution________0.159_______________ 6