been fully titrated. II. Discussion of Titration of Cola The objective of part B was to conduct a pH titration of phosphoric acid in a cola drink with a standardized base and then to determine the concentration of phosphoric acid in a cola drink and to determine the acid dissociation constants of phosphoric acid. We did this by using a LabQuest to log the titration of the cola sample with 0.01 M NaOH. At each interval we recorded the pH and volume delivered, gradually creating a polyprotic acid titration curve until we reached a pH of 10. Observed from the graph of the pH versus volume NaOH added, the first equivalence point occurred when 4.5 mL of NaOH was added with a pH of 5.08 and
the second equivalence point occurred when 14.5 mL of NaOH was added with a pH of 8.45. We know these are the equivalence points because the change in pH over the change in volume ratios were highest at these two points, indicating that they have the largest slope. The concentration of H 3 PO 4 was then determined using the first equivalence point, giving us a value of 2.25 M. We then used this concentration in an ICE table to find the final concentrations of the dissociation reaction and then to determine the value of K a1 . [ H 3 PO 4 ] = 0.0095 mol NaOH 1.00 L NaOH ∗ .0045 L NaOH ∗ 1 mol H 3 PO 4 1 mol NaOH = 4.275E-5 mol H 3 PO 4 = 4.275 E [H 3 PO 4 ] [H 2 PO 4 - ] [H + ] I 0.00214 0 0 C -10 -2.87 +10 -2.87 +10 -2.87 E 7.89*10 -4 0.00135 0.00135 K a1 =(0.00135 M) 2 /(7.89*10 -4 M) = 0.0023 pK a1 = -log(0.0023) = 2.64
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- Fall '12
- Sodium hydroxide, Cola Drinks