Introduction The purpose of this experiment is to determine the Ka value using the pH titration curve that we derive We are going to monitor the changes in pH that occur during the titration of a weak polyprotic acid with a strong base At the equivalence point one should expect to see a dramatic change in pH as the solution goes from acidic to strongly basic NaOH added mL Phosphoric Acid pH NaOH added mL Catherine Callahan Chem 112 Lab 99LL Data 0 02M NaOH Initial Buret reading 0 mL Initial pH of citric acid 2 64 Citric Acid pH 2 8 3 4 4 2 4 6 5 0 5 4 5 8 6 4 6 8 7 2 8 2 10 4 10 6 10 8 11 2 11 4 11 6 12 0 8 18 7 23 5 27 9 32 34 8 46 49 50 50 56 4 58 7 60 74 92 5 136 5 145 2 2 2 2 8 3 4 4 0 4 6 5 2 5 8 6 4 7 0 7 6 8 2 8 8 9 4 10 0 11 6 12 2 12 8 13 4 0 12 5 17 0 18 3 18 8 19 5 20 75 23 0 29 5 36 8 38 5 40 2 41 8 44 9 55 25 60 5 62 4 66 75 Equivalence point 50mL Ka 10 7 7 1 99526231E 8 Given Ka 4 0 x 10 7 at 25 degrees celsius Molarity mol solute volume solution in liters Citric Acid M 1 995E 8 123 5mL 1000mL 1 61538462E 7 Equivalence points 19 5mL 40 2mL Ka1 10 5 2 6 31E 6 Given Ka 7 5 x 10 3 at 25 degrees celsius Ka2 10 8 8 1 58489319E 9 Given Ka 6 2 x 10 8 at 25 degrees celsius Molarity mol solute volume solution in liters Phosphoric Acid M1 6 31E 6 mol 66 75mL 1000mL 9 45E 5 Phosphoric Acid M2 1 58E 9 mol 66 75mL 1000mL 2 3670412e 8 Discussion The difference between the given Ka and the Ka we derived is largely due to experimental error There are several factors that influenced mistakes in this experiment including misreading the volume of titrant and marking the values down before pH had stabilized as well as a leaky buret In addition the pH was rapidly changing at times and therefore it is possible inaccurate data was recorded
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