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Identification of an Unknown Weak Acid

Extracts from this essay...


Analytical Chemistry Laboratory 319 Experiment 7: Identification of an Unknown Weak Acid Formal Report Bonnie LaPierre Drawer G-1 Lab Date: Thursday March 13, 2008 INTRODUCTION: One of the more common reactions seen in chemistry is the acid/base reaction. The most common result of an acid/base reaction is the formation of water and a salt. Such a reaction can be highly useful in the analysis of an unknown acid or base. By titrating the unknown analyte with a standard reagent, several characteristics of the unknown can be determined, as well as the identity of the analyte itself. One important aspect of an acid/base reaction is the pH of the solution. Since an acid/base reaction is often performed by titrating an acid with a basic solution (or vice versa), monitoring the pH during the titration shows how the acidity is changing as reagent is added. By recording these pH values during the course of the reaction, a titration curve can be generated; from this curve, several characteristics of the reaction can be determined such as equivalence point(s), pKa(s), and end point(s), etc. In this experiment, the objective was to identify an unknown weak acid. This was accomplished by titrating the unknown acid with a standardized base.


Conc. NaOH (M) 1 0.7024 0.03445 0.09984 2 0.7026 0.03465 0.09929 3 0.7026 0.03465 0.09929 Average Conc. (M) StDev %RSD 0.09947 3.16E-04 0.3181 Using the mass of each KHP sample and the volume of NaOH used in the titration, the concentrations of each sample and, ultimately, the average concentration of NaOH was determined. Sample 1: Sample 2: Sample 3: Average: DATA: The data obtained from the titration is as follows: NaOH Added (mL) pH 0.00 1.87 0.85 1.88 2.00 1.91 2.99 1.95 4.00 2.00 5.00 2.05 5.99 2.11 6.95 2.18 7.90 2.25 8.60 2.30 9.00 2.35 9.50 2.40 10.10 2.48 10.61 2.55 11.05 2.62 11.31 2.68 11.71 2.77 12.06 2.86 12.35 2.97 12.80 3.14 12.96 3.29 13.38 3.68 13.60 4.10 13.80 4.34 14.20 4.65 14.49 4.80 15.00 5.02 15.49 5.14 15.80 5.22 16.31 5.32 16.81 5.42 NaOH Added (mL) pH 17.50 5.53 18.01 5.63 19.05 5.76 19.85 5.86 20.70 5.97 21.59 6.10 22.41 6.21 23.32 6.36 23.98 6.47 24.65 6.61 25.40 6.76 25.60 6.86 25.99 7.05 26.31 7.24 26.41 7.31 26.50 7.41 26.60 7.52 26.78 7.70 26.85 7.98 27.00 8.63 27.10 9.71 27.20 10.13 27.30 10.36 27.40 10.51 27.50 10.64 27.60 10.71 27.70 10.80 27.85 10.87 27.95 10.93 28.05 10.98 28.15 11.01 Using this data, a titration curve and its first derivative were generated using Microsoft Excel.


Therefore, it was determined that the unknown acid in question was maleic acid. Of all the possibilities, its pKa values and molecular mass most closely corresponded to the experimentally observed values of the unknown. Both the standardization of the NaOH and the observed pKa values show satisfactory precision. Much patience was spent in obtaining barely discernible shades of pink during the NaOH standardization, and the data obtained reflects this. However, if any amount of the mass of KHP weighed was lost in transferring to solution, the resulting concentration values would be skewed. Extra time was also spent in obtaining as many points as possible for the titration curve of the unknown acid. The higher standard deviation of the pKa1 values may have occurred in the interpolation of the pH values for A, B, and C of the first equivalence point, as this region was not quite as steep as the region of the second equivalence point, making interpolation more difficult. The pH meter was calibrated prior to analyzing the unknown acid, and therefore did not likely introduce any error. Care was taken to rinse all glassware before use in preparation or titration of the unknown acid. However, if any external chemicals were introduced from the glassware used, the pH values would be skewed.

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