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Abstract: In this experiment, examining the equivalence point in a titration with NaOH identified an unknown diprotic acid. The molar mass of the unknown was found to be 100.78 g/mol with pKa values of 2.6 and 6.6. The closest diprotic acid to this molar mass is malonic acid with a percent error of 3.48%. Introduction: The purpose of the experiment was to determine the identity of an unknown diprotic acid. The equivalence and half-equivalence points on the titration curve give important information, which can then be used to calculate the molecular weight of the acid. The equivalence point is the moment when there is an equal amount of acid and NaOH. Knowing the concentration and volume of added NaOH at that moment, the amount of moles of NaOH can be determined. The amount of moles of NaOH is then equivalent to the amount of acid present. Dividing the original mass of the acid by the moles present gave the molar mass of the acid. In this particular titration, there were two equivalence points as the acid is diprotic. Consequently, the titration curve had two inflection points. The acid dissociated in a two-step process with the net reaction being: H2X + 2 NaOH Na2X + 2 H2O
This was important to take into consideration when calculating the molar mass of the diprotic acid. If the first equivalence point was to be used, the ratio of acid to NaOH was 1:1. If the second equivalence point was used in the calculations, the ratio became 1:2 as now a second set of NaOH molecules reacted with the acid to dissociate the second hydrogen ion. The titration curve also showed the pKa values of the acid. This happened at the half-equivalence point where half of the acid was dissociated to its conjugate base (again, because of the diprotic properties of the acid, this happens twice on the curve). The Henderson Hasselbalch equation pH = pKa+log(A-/HA) shows that at the half-equivalence point, the pKa value equaled the pH and was visually represented by the flattest part of the graphs. Discussion: The titration graph showed that the data was consistent with the methodology and proved to be an precise execution of the procedure and followed the expected shape. One possible source of error was the actual mass of the acid solid. While transferring the dust from the weigh boat to the solution, some remained in the weigh boat this could have altered the molar mass calculations and shifted the final the final mass lighter than actual. The Vernier pH method was definitely a much more concrete method of interpreting the results. It was possible to see which addition of NaOH gave the greatest increase in pH ( greatest 1st derivative of the titration graph). The relying solely on the indicator color would make it very difficult to judge at which precise point the color shifted most, as the shift was a lot more gradual compared to the precise numbers. This may have been a more reliable method if there was a device like a colorimeter to observe the precise wavelength of the solution.
The unknown was most likely malonic acid as both the calculated molar mass pKa values are closest to it than the other diprotic acids. The judgment would be questionable if molar mass was the only indicator as the calculated mass may have been a more severe skew of a different diprotic acid. However, the pKa values confirmed that the closest acid to the one identified is malonic acid. The color shifted from yellow to an almost clear color right before it switched to blue. This happened right around the equivalence point. That would make the pKa value of Bromocresol Green to be around 4.5. The pKa value of the other indictor was somewhere around 9.5-9.7.