b. The pK a1 for H3PO4 is 2.22 pH and the pK a2 for H3PO4 is 7.12 pH. The pk a ’s for the phosphoric acid titration curve are simply just the pH listed on the graph at the midpoint 1 & midpoint 2. The pH for midpoint 1 is found from finding the equivalence point 1 on the graph and then taking half of that to get halfway between the start of the titration and the equivalence point. The pk a for midpoint two is found the same way however it is halfway between the first and second equivalence points. At the midpoint, the Henderson-Hasselbalch equation simplifies to pH=pK a . As stated in the lab manual the pk a for each successive proton dissociation is the pH at the midpoint of the reaction for that specific proton; in other words the midpoint is where half of those protons are titrated. The pH for the pK a1 listed in the book is 2.1 and the pH for my pKa1 is 2.22.The pH for the pK a2 listed in the book is 7.2 and the pH for my pK a2 is 7.12. The pk a ’s listed in the book are very close to those that I found experimentally meaning there was little error in the experiment because they are very similar. c. The K a1 for H 3 PO 4 is 6.03x10^-3 and the K a2 for H3PO4 is 7.6x10^-8. The K a ’s were found by finding the midpoint’s on the graph for the 2 equivalence points. From getting the midpoints we were able to get the pH at the two specific points. There is a relationship between K a and pK a . PK a is the pH. Then to find the K a you can take the negative log of the pH. For midpoint 1 the pH was 2.22 so to find K a I did 10^ -2.22 . I did the same thing for the midpoint to but with the pH for the midpoint two which was 7.12. I know that the K a ’s that I calculated are close to the real K a values because The K a1 I found in the book for H 3 PO 4 is 7.5x10^ -3 and the K a2 I found is 6.2x10^ -8 . The fact that they are fairly close to each other means that the equivalence points that I found in my experiment are fairly close to the actual equivalence points.
d. The H 3 PO 4 concentration in Cherry Cola is 0.0544 M. I found this molarity the same way I found the initial concentration of H 3 PO 4 with the phosphoric acid titration curve. The first step I took in the process of determining the molarity of H 3 PO 4 in the Cherry Cola was finding the two equivalence points. I found the equivalence points on the graph, you can easily find them because they occur when there is a jump in the pH. By looking at the
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