uncch102L_exp16 - Experiment 16 Identification of an...

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Unformatted text preview: Experiment 16 Identification of an Unknown Acid Summer 2010 I HAVE USED THIS LAB! IF YOU USE MY LABE VERBATUM, YOU WILL BE CAUGHT! Point Summary (See Blackboard for detailed grading rubric) Superior Excellent Satisfactory Fair Poor Omitted Introduction Purpose of Report Goals of Experiment Materials and Methods Results and Discussion Description of data Data Tables Data Table Titles Graphs Figure Captions Sample Calculations Systematic Error Random Error Discussion of discrepancies Other Unknown Identification TOTAL POINTS TA Comments/Suggestions: CHEMISTRY 102L REPORT TEMPLATE Identification of an Unknown Acid Introduction EXPT. 16 The purpose of this report is to determine the identity of an unknown acid species by analyzing a titration curve and calculating the molar mass of the acid. The purpose of the experiment was to collect pH readings while titrating an unknown acid with a standardized NaOH solution. The equivalence points were determined to give aid in identifying the molar mass for the acid which, in turn, help to determine the identity of the acid. Materials and Methods The procedure for Experiment 16: Identification of an Unknown Acid was acquired from the Fall 2010 UNC-CH CHEM-102L Lab Manual. A clean 50-mL buret was conditioned and filled with the NaOH solution that was standardized in the previous lab. To the nearest milligram, 0.3-0.4 g of assigned unknown acid was weighed into a clean 100-mL beaker. The weight was recorded The card was not highlighted so the unknown acid was dissolved in approximately 50 mL of distilled water and the solution was allowed to stir on a stir plate until completely dissolved. The clean, dry pH electrode was inserted into the solution and the initial pH was recorded. NaOH was slowly tirated into the beaker of acid and pH readings were recorded every 0.3-0.5 change in pH until the solution was at a pH of approximately 12. The procedure was followed precisely excepting that it was done individually and only one unknown acid titration was completed. Results and Discussion Acid-base titrations can aid in determining the identity of an unknown acid or base, in this case an acid. The base is slowly titrated into the acid and the pH is monitored and recorded. Based on the titration curve (volume vs. pH) acquired during experimentation, the moles of both acid and base can be determined at the equivalence point. A first derivative analysis of the titration curve is the most accurate way to determine the volume at which an equivalence point has been reached. The first derivative analysis indicates the greatest changes in slope with inflection points. The peak of an inflection point corresponds to the volume of the base added at the equivalence point as seen below in Figure 1. Figure 2 shows the labeled equivalence point and the midpoint. The equivalence point for a monophonic acid should have a pH of about 7.0 and the volume as determined by first derivative analysis. The midway point is the point of the graph where the volume of the base is half of that at the equivalence point. Figure 1. Titration of unknown acid from pH 1.8 to pH 11.9 with first derivative analysis Figure 2. Equivalence point and midpoint for unknown acid titration Figures 1 and 2 can be used to determine the molar mass and the pKa value for the unknown acid, which are then used to identify the acid. At the equivalence point, the moles of acid are equal to the moles of base. The first derivative analysis of the graph gives us the volume of NaOH at the equivalence point. The concentration of the NaOH solution is known and when the molarity is multiplied by the volume the number of moles of NaOH, and therefore the unknown acid, is determined. The mass of unknown acid that was used is also known and can be divided by the number of moles to determine the molar mass of the unknown acid. At the midpoint, the pH is equal to the pKa. The results for the molar mass and the pKa can be found in Table 1. The mass of the unknown acid in the 100-mL beaker was 0.324 grams and the initial pH was 1.8. The total volume of 0.0845 M NaOH used to reach the final pH of 11.9 was 50.2 mL and was 27.90 mL at the equivalence point. Following the guidelines provided in the rubric, show a sample calculation for the determination of the molar mass of the unknown acid. Table 1. Experimental and Accepted Values for Molar Mass and pKa of Sodium bisulfate monohydrate Experimentally Determined Literature Value Percent Error Molar Mass 0.04% error 138.01 138.07 pKa 2.00 1.92 4.2% error According to the results presented in Table 1, the identity of the unknown acid is sodium bisulfate monohydrate. The calculated molar mass was close to the accepted value of the molar mass, according to the lab manual, so there was no significant source of error in determining the molar mass. Measurements and factors affecting the molar mass of the acid were the experimentally determined concentration of the NaOH solution, the mass of the unknown acid used in the titration, and the volume of the NaOH solution added to achieve equilibrium. If any of these numbers were slightly off by rounding error or by an error in measurement the molar mass would be slightly off. The error in pKa could have been affected by the temperature in the room. The pKa value is dependent on temperature. The pKa values given in the lab manual, the value used at the literature value, were likely determined at the standard temperature of 25C. The temperature of the room was not measured in this experiment, but a slight variation of temperature from 25C would have unpredictable results on the entire system leading to a calculated pKa value different than the literature value. To fix this in the future, the temperature of the room could have been monitored and set at 25C. ...
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This note was uploaded on 05/15/2011 for the course CHEM 102L taught by Professor N/a during the Fall '07 term at UNC.

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