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Unformatted text preview: 1 Analysis of Ni Complex Lab Worksheet By reviewing the material in sections 3.1-3.5 in your text, and reading the lab, you should be able to get a handle on these calculations. Although the compound used for these calculations is fictional, every calculation that you perform is identical to those required in the lab. Since each week of this lab is worth 20 points, it is worth investing the time to figure out what you need to do prior to the lab. You will be permitted to use your own set of calculations for this worksheet as a reference when performing the lab write-up; you will not be permitted to use another student's work. Note that the WEEK 2 calculations are more involved than WEEK 1. WEEK 1 Practice Sheet You are given 5.0 g of a hypothetical compound AwBxCyDz in some water. Upon dissolving, the compound splits apart into A(aq), B(aq), C(aq) and D(aq). B is a base that undergoes the following reaction with HCl: B + 2 HCl H2B2+ + 2ClIn order to determine the number of moles of B in the compound you perform a titration. First you dissolve 0.500 grams of the compound in 20 mL of water. You then perform the titration and find that it takes 20.00 mL of 0.0100 M HCl to reach the end point of the titration. The molar mass of B is 500.0 g/mol. Based on the information from above, complete the following: Moles of HCl used in titration: _____________________________ (Answer: 2.00 x 10-4 mol) Moles of B titrated: ______________________________________ (Answer: 1.00 x 10-4 mol) Mass of B titrated:_______________________________________ (Answer: 5.00 x 10-2 g B) Mass % of B in the compound:___________________________________(Answer: 10.0% B) Moles of B in 100 grams of the compound: _________________________ (Answer: 0.0200 mole) 2 WEEK 2 Practice Sheet Reminder: There are 0.0200 moles of B in 100 grams of the compound: AwBxCyDz (Week 1). You dissolve 0.250 g of your compound (AwBxCyDz) in exactly 10.00 mL of water producing 10.00 mL of solution. Using the photometer and standard solutions, you find that the concentration of A is 2.00 x 10-2 M. The molar mass of A is 750.0 g/mol. Moles of A in the solution sample:________________________________(Answer: 2.00 x 10-4 mol A) Mass of A in the solution sample: _________________________________ (Answer: 0.150 g A) Mass % of A in the compound: _______________________________(Answer: 60.0% A) Moles of A in 100 grams of the compound: __________________________ (Answer: 0.0800 mol A) The compound, AwBxCyDz has an oxidation number of 0 (zero). The oxidation number of A is +1, the oxidation number of B is +2, the oxidation number of C is 3, and the oxidation number of D is 0. The molar mass of C is 500.0 g/mol. Moles of A in 100 grams of the compound: 0.0800 mol A Moles of B in 100 grams of the compound: 0.0200 mol B Moles of C in 100 grams of the compound (use oxidation numbers): _________ (Answer: 0.0400 mol C) Grams of C in 100 grams of the compound:_______________________________ (Answer: 20.0 g C) Mass % of C in the compound:__________________________________ (Answer: 20.0% C) The molar mass of D is 166.7 g/mol. Mass % D in the compound:_________________________________ (Answer: 10.0%) Mass of D in 100 grams of the compound:____________________________ (Answer: 10.0 g D) Moles of D in 100 grams of the compound:____________________________ (Answer: 0.0600 mol D) Formula of compound: ___________________________ (Answer: A4BC2D3) 3 ANALYSIS OF A NICKEL COMPLEX Copyright: Department of Chemistry, University of Idaho, Moscow, ID 83844-2343; 2008. INTRODUCTION This lab will span the next two lab sessions. Throughout both weeks you will be analyzing a coordination complex (a compound) containing nickel. Coordination complexes are an important class of compounds. They are found in paint pigments, colored glass, and in both natural and synthetic gemstones. Some complexes perform crucial biological functions. Coordination complexes consist of a metal ion that is attached (bonded) to elements and/or other compounds called ligands. In this lab the ligands that will be attached to the nickel ion are: H2O, SO42-, and ethylenediamine (abbreviated "en"). Ethylenediamine is a weak base, with the formula H2N-CH2CH2-NH2. For background on coordination complexes, you may want to consult Section 22.3 in your text. The complex that you will analyze was prepared by reacting ethelyenediamine, NiSO4 and water, forming a product with general formula: [Niv(en)w (H2O)x (SO4)y ] z H2O The amount of the reactants determines how many of each ligand will attach to the nickel ion and hence the color and makeup (the values of v, w, x, y and z) of the product. Following the procedure outlined below, you will analyze a compound of unknown composition; i.e. you will not know the values of v, w, x, y, and z. Through careful work using a variety of techniques that you have learned this semester, you will be able to determine a likely chemical formula of the product. During the first week you will determine by titration, the mass percentage of ethylenediamine in your compound, which will then allow you to calculate: w= moles of ethylenediamine in the compound The procedure for the second week will outline how you can determine/calculate: v = moles of Ni2+ y = moles of SO42- in the compound x+ z =moles of H2O in the compound (note that you only will know the sum of x+ z) Once you have all of this information, you should be able to determine the formula of the complex. In this lab you will have to perform several calculations of mass percentage. Be sure to review Sections 3.1-3.5 of your text. 4 PROCEDURE (Week 1) Titration of ethylenediamine with standard HCl solution This first procedure is to determine the number of ethylenediamine (H2N-CH2-CH2-NH2) molecules attached to the nickel ion(s) in your compound. Be sure to pay attention to the stoichiometry of the titration reaction! 1. Green aqueous Ni2+ ion is released into the solution when the complex dissociates. This masks the endpoint of the indicator. To account for the color masking, prepare an artificial endpoint solution. Add 5 drops of the indicator to a titration flask with 0.1 g of NiSO46H2O dissolved in 20 mL of the standardized acid. Use the color of this solution as your permanent end point indicator when performing the titration. If the color fades while performing the titration, continue adding HCl. Weigh a sample (about 0.15 g) of the assigned complex and add it to an Erlenmeyer flask (be sure to record the exact mass). Add 10 mL of water and completely dissolve your sample. After the entire sample has dissolved, add 5 drops of indicator, bromcresol green, and titrate with a standardized acid to the yellow end point. Repeat the titration at least twice or until you have consistent data. Your compound dissociates very slowly in water according to the equation below; therefore be patient enough to ensure that the sample is completely dissolved before titrating to the permanent, lasting end-point. Dissociation Equation [Niv(en)w (H2O)x (SO4)y] z H2O (aq) Ni(H2O) 2+ + H2N-CH2-CH2-NH2 Titration Equation H2N-CH2-CH2-NH2 + 2 HCl H3N+-CH2-CH2-N+H3 + 2 Cl- 2. 3. 5 PROCEDURE (Week 2) Colorimetric Analysis for Ni 2+ This procedure is used to determine the number of moles of nickel ions in your compound. 1. The LabQuest Colorimeter needs to be powered for about 5 minutes before using so do this step before preparing your solutions. Plug the LabQuest Colorimeter into one of the LabQuest sensor ports and turn on the instrument. 2. Load a 25 mL burette with about 15 mL of NiSO4 6H2O stock solution (be sure to record the molarity of the stock solution in the lab report). Load another burette with deionized water. 3. Use the given molar concentration of the NiSO4 6H2O solution to determine the molar concentration of Ni2+ that will be in each test tube given in the Table below once all reagents have been mixed together. 4. Clean and label five test tubes as shown in the Table below. Deliver the volumes of nickel stock solution and water shown in the Table. Do not add the ethylenediamine at this time. Just prior to making each absorbance reading, add 5.0 mL of 10 % ethylenediamine from a preset pump dispenser to the test tubes and mix. mL Ni 2+ stock solution 0 1.0 2.0 3.0 4.0 Tube A B C D E mL deionized H2O 5.0 4.0 3.0 2.0 1.0 mL en 5.0 5.0 5.0 5.0 5.0 Molar Concentration of Ni2+ in solution 5. You will not experimentally determine the wavelength of maximum absorbance in this lab. Be sure one of the four green wavelength lights is on. The wavelength of maximum absorbance in this experiment is 565 nm. Using the front panel arrow keys on the colorimeter choose 565 nm. 6. Calibrate the colorimeter. Add the ethylenediamine to test tube A (the blank). Rinse and fill a cuvette with the blank. Place it in the colorimeter set at the wavelength of maximum absorbance. Press the CAL button on the front of the colorimeter. Release it when the red LED begins to flash. When the red light stops flashing, the colorimeter is calibrated and ready to use. 7. Using the stylus (never touch the LabQuest screen with your fingers): touch Sensors, Data Collection, choose Events with Entry in the Mode box. Highlight the word Event in the Name: box and replace it with Concentration. Put M in the Units box. OK. 6 8. Add the ethylenediamine to the test tube containing the most dilute solution and mix. Rinse (using the solution that will be analyzed) and fill the cuvette with the solution. Place it in the colorimeter. Touch the begin data collection button (triangle) on the bottom left of the screen. When the absorbance reading stabilizes, touch the KEEP button located just to the right of the data collection button. Type in the concentration of the solution that is in the cuvette (don't include units). OK. 9. Pour your samples back into the original TT after you have measured each absorbance and discard them in the waste container. 10. Working in order of most dilute to most concentrated of the remaining solutions, rinse and then fill a cuvette with the solution that will be analyzed. Place the cuvette in the colorimeter. When the absorbance reading stabilizes, touch the KEEP button (don't touch the stop data collection button (square)), and enter the concentration of the analyzed solution. Repeat until the absorbance of all solutions has been determined. Remember to add the ethylene to the test tube, mix, and rinse the cuvette with the solution that will be analyzed. 11. When the absorbance of all prepared solutions has been determined, touch the stop data collection button (square) located on the bottom left of the screen and save your data. 12. Your calibration curve should be linear. Use the LabQuest to determine the equation of the line. 13. Prepare a solution sample of your Unknown. Weigh between 100 to 150 mg of your compound into a 10 mL volumetric flask (record the mass exactly). Add 4 mL of water to dissolve it. If necessary add 3 M HCl one drop at a time to help your sample dissolve. Once dissolved, add 5.0 mL of 10% ethylenediamine to the 10 mL volumetric flask. Dilute the flask to the 10 mL volume mark with water. Mix thoroughly. To make filling the cuvette easier, pour about 5 mL of this solution into a large test tube. 14. Rinse and fill a cuvette with the solution containing your unknown. Place it in the colorimeter. Touch the Sensor icon on the upper left of the LabQuest. Record the absorbance once the reading has stabilized. 15. Print out a copy of the calibration curve for each lab partner. Each lab report must have a LabQuest generated printout of the calibration curve attached to it. The printout must show the graph, the information needed to generate the equation (slope and intercept) for the line, and the correlation reading. This should all show up automatically on the printout. Be sure that the names of all lab partners are entered in the footnote and that the date box is checked. 16. Delete all saved files before returning your LabQuest to Lab Services. 7 Determination of sulfate content of your compound In order to determine the number of sulfate ligands attached to the nickel ion, you need to consider the following information. The overall oxidation number on the complex, [Niv(en)w (H2O)x (SO4)y ] z H2O, is zero. The oxidation number of water is zero. The oxidation number of the sulfate ion is -2. The oxidation number of ethylenediamine, en, is zero The oxidation number of nickel in the complex is +2. The % en and % Ni2+ may be used to calculate the number of moles of en and Ni2+ present in 100 grams of the compound. You then should be able to calculate the number of moles of sulfate in 100 grams of the compound. Water Determination At this point, you know the mass percentages of all of the ligands except for the water. Keep in mind that the percentages must add up to 100% for the compound. Use this fact to first determine the percentage of water in the compound. Empirical Formula Determination The number of moles of each component (i.e., ions, ligands) in 100 grams of the compound can be determined using mass percentages and molar masses. At this point we will deviate a bit from the method used in the textbook to determine empirical formulas of compounds. Because the sample may include moisture from the air as well as water from the actual compound, the mole ratio of water in the compound to the other components in the compound may not be a whole number. Instead of dividing the moles of all components by the smallest number of moles, divide the moles of all components (including water) by the smallest of the number of moles of the remaining components (nickel, sulfate, ethylenediamine). If you need to multiply by a number to get the smallest set of whole number subscripts on nickel, sulfate, and ethylenediamine, do so. Multiply the subscript on wat...