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UMass Amherst CHEM 112 - Chem 112 Lab Report 5 (1)

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Chem 112 PH Titration Curves Idealized Purpose: The purpose of this lab is to see the results of titration with two acids and see what a titration curve looks like and how to interrupt it and to determine dissociation constants for phosphoric and citric acid.Calculations: Citric acid equivalence point ● V1=19.33mL (1/3 * V3) ● V2=38.63mL (2/3* V3) ● V3=58mL (from graph) ● V(midpoint)1= 9.67mL (½ * V1) ● V(midpoint)2=28.98mL (½ (V1+V2)) ● V(midpoint)3=48.32mL (½ (V2+V3)) At midpoint, pH=pKa. Then, pKa=-log[H+] so, [H+]= 10^-pKa this is how to find Ka. ● pH1(pKa1)=2.9 → [H+]=.00126 ● pH2(pKa2)=4.5 → [H+]=.000032 ● pH3(pKa3)=6.4 → [H+]=.00000039 ● Ka1= 1.26E-3 ● Ka2= 3.2E-5 ● Ka3= 3.9E-7 Phosphoric acid equivalence point ● V1=18.1mL (from graph) ● V2=36mL (from graph) ● V3=54.3mL (3*V1) ● V(midpoint)1=9.05 (½ * V1) ● V(midpoint)2= 27.05 (½ (V1+V2)) ● V(midpoint)3= 45.15 (½ (V2+V3)) At midpoint, pH=pKa. Then, pKa=-log[H+] so, [H+]= 10^-pKa this is how to find Ka. ● pH1(pKa1)=2.9 → [H+]=.00126 ● pH2(pKa2)= 6.8 → [H+]=.00000016 ● pH3(pKa3)=11.1 → [H+]=.0000000000079 ● Ka1= 1.26E-3 ● Ka2= 1.6E-7 ● Ka3= 7.9E-12 Discussion: In the experiments the third equivalence point on the graph for Phosphoric Acid is not visable because it is so basic at this point similar to how the first 2 equivalence points on the graph for Citric Acid are not visible because it is so acidic. By looking at the graphs of both acids, neither shows all three of the acids equivalence points so in order to find them we had to calculate what they would approximately equal with what we know about the other equivalence points. This can account for if there were some differences among group members in the Ka values since they were mostly done by interpretation of the graph and then calculations. The pKa values given by the procedure for both acids differ from the ones we obtained in our experiment this could be caused by an experimental error or human error, while none of the values match up perfectly, the powers seen in the results for citric acid and the procedure match up well.Results: The purpose of this experiment was to determine the dissociation constants for phosphoric and citric acid. The experimental results were not ideal because it is difficult to get precise measurements of all equivalence points due to highly acidic or basic conditions of the titrations. We were however, able to graph and produce accurate titration curves and interpret them for further


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UMass Amherst CHEM 112 - Chem 112 Lab Report 5 (1)

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