Prelab for Phosphoric Acid Titration Curve - Prelab for Phosphoric Acid Titration Curve Chemistry 112 Lab Section 005 1 The inflection point of a

Prelab for Phosphoric Acid Titration Curve - Prelab for...

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Prelab for Phosphoric Acid Titration Curve Chemistry 112 Lab Section 005 October 29, 2014 1.The inflection point of a titration curve is also the equivalence point 2.a. Why is the third equivalence point of phosphoric acid not detectable in water? b. Why does the cola soft drink need to be decarbonated before titration? Be very specific. 3.In conducting a titration of 75.00 mL of a certain triprotic acid with 0.0800 M NaOH, the following information was recorded: A graph of “Volume NaOH added (mL)” vs “pH”.Using the graph, calculate the apparent concentration of this triprotic acid and the dissociation constants for all proton dissociation steps (Ka1, Ka2, and Ka3). Approximations of points on the graph1stequivalence point (12.5 ml, 4.9 pH)2ndequivalence point (25.5 mL, 8.1 pH)Midpoint 1 (6.0 mL, 3.62 pH)Midpoint 2 (18.5 mL, 6.2 pH)Midpoint 3 (31.25 mL, 9.5 pH)pKa1 = 3.62pKa2 = 6.2pKa3 = 9.5Ka1= 103.62 = 2.40 10Ka2= 106.2 = 6.3 10Ka3= 109.5 = 3.2 10It took 12.5 mL to reach first equivalence point and 13 mL (25.5 mL – 12.5 mL) to reach second equivalence point. Average = 12.75 mL per equivalence point ((12.5 mL + 13 mL)/2))0.0800 mol/L NaOH x 0.01275 L NaOH = 0.00102 mol NaOHMoles of triprotic acid = moles of NaOHTherefore there are 0.00102 mol triprotic acid0.00102 mol triprotic acid/0.07500 L triprotic acid = 0.0136 M triprotic acidThe concentration of this triprotic acid is 0.0136 M. 4 7 10

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