The most prominent source of error in this experiment

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close together, the anomaly (trial 2) was disregarded. The most prominent source of error in this experiment was the fact that extremely small quantities of KHP were used in the titration. As the mass of the samples were being measured, the mass was constantly changing with the air currents in the room. Although we waited an appropriate amount of time for the balance to give a definite reading on the mass, it is likely that there was still some unaccountable error in the mass readings (especially for trial 2). The purpose of the second part of the experiment was to make a titration curve for phosphoric acid. Using this curve, the various Ka/pKavalues of the phosphoric acid (polyprotic) could be calculated. 20.0 mL of cola were used for this titration with a starting pH of 2.42. The NaOH was added in approximately 1.0 mL quantities. After around 30 mL of the titrant was added, the titration was ended with a final pH of a little more than 10. Using the data from the titration, a function was also made to represent the slope of the titration curve. On a regular titration graph, the volume of added titrant is the x axis and the pH of solution is the y axis. Therefore, the x axis for the derivative function was the average of the volume and the change in pH divided by the change in volume as the y axis. The derivative function helped with identifying certain positions in the graph. For example, the peaks of the derivative function identified where the inflection points were located on the titration curve. The inflection point is where the slope changes from positive to negative, or essentially where the equivalence points were. Once the equivalence points where known, the pKacould be identified. The pKais equal tothe pH after half of the acid was titrated by the strong base. By using the locations of the equivalence points, the half-equivalence points could be identified to find the various pKavaluesfor phosphoric acid. Because phosphoric acid is a polyprotic acid, it has multiple Ka/pKavalues. This simply means that phosphoric acid has numerous hydrogen ions to donate in solution, and will experience several equivalence points during the titration. By using the methods described in the previous paragraph, the pKavalues were located on the graph (as shown by the black lines). The first pKa value that we found was at pH = 2.9 (4.5 mL NaOH added), and the second pKavalue was at pH = 7.0 (14.5 mL NaOH added). The resulting Kavalues were 1.3 X 10-3and 1.0 X 10-7respectively. The accepted values for the first and second calculated pKa/Kavalue was 2.2/7.1 X 10-3and 7.2/6.3 X 10-3, indicating that there was a source of error in the experiment. This source of error is random because the measured Ka1was too small but the measured Ka2was too large. The calculation without the graph for the Ka of phosphoric acid yielded a value of
2.1 X 10-2. This value is significantly higher than the calculated Kavalues because it doesn’t account for the fact that phosphoric acid is polyprotic. The value 2.1 X 10-2is the overall Kawhen

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