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Exp 3 Lab Report - The examination of electrochemistry and...

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The examination of electrochemistry and application of redox reactions in determining the ascorbic acid content in a vitamin C tablet Jubilee Ngo TA: Christine Huang Section: A00 Chemistry 100A at University of California, San Diego 9500 Gilman Drive, La Jolla, California 92093 17 July 2006 Abstract The application of electrochemistry in constructing electrochemical cells was observed in the first part of this experiment. Initially, the cell potentials of several Daniel cells consisting of zinc sulfate solution, copper sulfate solution, copper strip (cathode), zinc strip (anode), and moistened filter paper (salt bridge) were determined using a pH/volt meter. Then, the cell potentials of concentration cells made of varying copper sulfate solutions were measured to investigate the relationship between concentration and potential. In the second part of the experiment, the mass of ascorbic acid, or vitamin C (C 6 H 8 O 6 ), in vitamin tablets was calculated using a series of redox reactions and a back titration. First, sodium thiosulfate was standardized with potassium iodate as the primary standard and its concentration was determined to 0.1316 ± 0.0013 M. Then, sodium thiosulfate was titrated with excess bromide, which was produced in a reaction of potassium bromate and potassium bromide mixture. The average mass of ascorbic acid in a 25-mL solution is 0.02625 ± 0.00137 g and due to many errors encountered throughout the experiment, the ascorbic acid content in a 673.5-g vitamin tablet is 701.6 g, which clearly impossible. Result Table 1. Cell potentials of Daniel cells Solutions (M) E cell (mV) None (baseline value) 0.20 0.10 M ZnSO 4 || 0.10 M CuSO 4 1077.8 0.10 M ZnSO 4 || 0.0010 M CuSO 4 1059.8 0.0010 M ZnSO 4 || 0.10 M CuSO 4 1049.8 Table 2. Cell potentials of concentration cells Solutions (M) E cell (mV) E cell (V) E cell = (-2.303 * RT / nF) * log Q 0.10 M CuSO 4 || 0.0010 M CuSO 4 57.4 0.0574 0.0596 0.10 M CuSO 4 || 0.00010 M CuSO 4 29.7 0.0297 0.0887 0.10 M CuSO 4 || 0.000010 M CuSO 4 28.7 0.0287 0.1182 0.10 M CuSO 4 || 0.0000010 M CuSO 4 41.0 0.0410 0.1478
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Table 3. Molar mass and concentration of each solution Na 2 S 2 O 3 5H 2 O Na 2 CO 3 KIO 3 KBrO 3 KBr Vit_C Molar Mass: 248.1841 105.98844 214.0010 167.0005 119.0023 176.1241 Mass used(g): 3.7219 2027 0.212 0.1518 0.1008 Volume(mL) 150 100.00 120.00 Conc(M) 0.10 0.00991 0.007575 Table 4. Standardization of sodium thiosulfate solution Flask1 Flask2 Flask3 KIO 3 used(mL) 25.00 25.00 25.00 KI(g) 1 1 1 (NNBE) 6MHCl(mL) 1 1 1 (NNBE) Initial buret reading(mL) 2.75 14.20 25.10 Colorless Final/Initial buret reading(mL) 14.15 25.07 36.62 Pale Yellow Starch(mL) 5 5 5 Blue Final buret reading(mL) 14.20 25.10 36.64 Colorless Na 2 S 2 O 3 used(mL) 11.45 10.90 11.54 KIO 3 reacted(mol) 2.477E-04 2.477E-04 2.477E-04 6H + + 6S 2 O 3 2- + IO 3 - I - + 3S 4 O 6 2- + 3H 2 O Na 2 S 2 O 3 reacted(mol) 1.486E-03 1.486E-03 1.486E-03 [Na 2 S 2 O 3 ] (M) 0.1298 0.1363 0.1288 Average [Na 2 S 2 O 3 ] (M) 0.1316 sd [Na 2 S 2 O 3 ] (M) 0.0041 Table 5. Percentage of ascorbic acid in sample and mass of ascorbic acid in Vitamin C tablet Vitamin tablet mass(mg) 673.5 Nominal ascorbic acid mass(mg) 500 Total volume of sample prepared(mL) 100.00
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