# 000750 the algebraic and graphical determination were

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were not the same as the literature value, but the value determined algebraically is much closer than the value determined graphically thus we can assume that calculating the Ka
algebraically is more accurate than calculating it graphically. Graphical analysis is not valid in determining the Ka for phosphoric acid, as can be seen in the error between the our graphically determined values and the literature values.An error that was involved in this experiment was our assumption that the room maintained a temperature of 25oC or 298K. This is important because Ka is dependent on temperature based on the Van’t Hoff equation and if this temperature was not maintained,then our Ka value would be slightly inaccurate. Another assumption we made was that allthe carbon dioxide was completely removed from the sample. If the carbon dioxide was indeed present, then the pH of our solution would be affected and the concentration of our hydronium ion would change along with our Ka value. We had to visually determine the pH on our titration curve, which is most likely not 100% accurate. NaOH was added in somewhat large increments of 1.0 mL.  If smaller increments were used then a more precise equivalence point and Ka. could have been determined.Discussion Questions1.The graph obtained from the titration date clearly illustrates that the reactions that occurred have two equivalence points and match up well with the actual graph for this dissociation, but visually gathering data from a graph will not beperfectly accurate because we have to estimate to an extent. Determining the volume for the equivalence and halfway point are very accurate, but with the pH, we have to estimate. This means that our H3O+concentration may be a little off when we calculate this from the pH we have visually determined, leading to a slightly inaccurate Ka expression. Since pH and pKa are equal at the halfway point, the Ka we determine directly from the pKa will be slightly off too.2.We must de-gas the cola sample prior to the titration to remove all the carbon dioxide from the cola. This must occur to prevent any side reactions in the titration, so that we may obtain a pure pH based on the phosphoric acid only. If carbon dioxide was involved, we would obtain a different value for our pH and pKa, and therefore, our calculations for the Ka value would be wrong.IV. ReferencesBottomley, L; Bottomley, L.A.; Chem 1211K/1212K, Chemical Principles Laboratory Manual, 2014-2015, p. 120-123.