Part II Test an Unknown Solution of SCN – 10. Obtain about 10 mL of the unknown SCN – solution. Use a pipet to measure out 5.0 mL of the unknown into a clean and dry 100 mL beaker. Add precisely 5.0 mL of 0.200 M Fe(NO 3 ) 3 and 40.0 mL of distilled water to the beaker. Stir the mixture thoroughly. 11. Using the solution in the beaker, rinse a cuvette twice with ~1 mL amounts and then fill it 3/4 full. Place the cuvette of unknown in the device (Colorimeter). 12. Determine the concentration of the unknown SCN – solution. a. Tap the Meter icon located on the top left-hand corner. b. Monitor the absorbance value. When this value has stabilized, record it in your data table. c. Tap the Graph tab. On the Graph screen, choose Interpolate from the Analyze menu. Tap any point on the regression cu rve (or use the ◄ or ► keys on LabQuest) to determine the concentration of your unknown SCN – solution. Record the concentration in your data table. Part III Prepare and Test Equilibrium Systems 13. Prepare four test tubes of solutions, according to the chart below. Repeat Steps 11 and 12 from Part II to test the absorbance values of each mixture. Record the test results in your data table. Record the [FeSCN 2+ ] in the table in Data Analysis section #3. Note : You are using 0.0020 M Fe(NO 3 ) 3 in this test. Test tube number 0.0020 M Fe(NO 3 ) 3 (mL) 0.0020 M SCN – (mL) H 2 O (mL) blank 3.00 0.00 7.00 1 3.00 2.00 5.00 2 3.00 3.00 4.00 3 3.00 4.00 3.00 4 3.00 5.00 2.00
The Determination of an Equilibrium Constant Los Angeles City College 10 - 5 REPORT SHEET EXPERIMENT 10 NAME: ____________ PRE-LAB EXERCISE For the solutions that you will prepare in Step 2 of Part I, calculate the [FeSCN 2+ ]. Presume that all of the SCN – ions react. In Part I of the experiment, mol of SCN – = mol of FeSCN 2+ . Thus, the calculation of [FeSCN 2+ ] is: mol FeSCN 2+ ÷ L of total solution. Record these values in the table below. Show your calculations in your lab notebook. Beaker number [FeSCN 2+ ] 1 2 3 4 (blank) 0.00 M POST-LAB REPORT DATA TABLE FOR PARTS I & II Beaker Absorbance [FeSCN 2+ ] 1 2 3 4 Unknown, Part II DATA TABLE FOR PART III Test tube number Absorbance 1 2 3 4
Chemistry 102 10 - 6 Los Angeles City College DATA ANALYSIS 1.(Part II) Use the calibration equation from Item 1 and the absorbance reading for your unknown solution to determine [SCN–]. 2.(Part II) Compare your experimental [SCN–], of your unknown, with the actual [SCN–]. Suggest reasons for the disparity. 3.(Part III) Use the absorbance values, along with the best fit line equation of the standard solutions in Part I to determine the [FeSCN2+] at equilibrium for each of the mixtures that you prepared in Part III. Complete the table and give an example of your calculations below. Test tube number 1 2 [FeSCN2+] 4.(Part III) Calculate the equilibrium concentrations for Fe3+and SCN–for the mixtures in Test Tubes 2–5 in Part III. Complete the table and give an example of your calculations. Test tube number 1 2 [Fe3+] [SCN–] 5.Calculate the value of Keqfor the reaction. Explain how you used the data to calculate Keq. 3 4 3 4
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