Prelab for Phosphoric Acid Titration Curve

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 of titration. 2. a. Why is the third equivalence point of phosphoric acid not detectable in water? The third equivalence point of phosphoric acid is not detectable in water because it occurs at such a high pH. The pH is too high because the third equivalence point is very small. b. Why does the cola soft drink need to be decarbonated before titration? Be very specific. The cola soft drink needs to be decarbonated because the carbon dioxide that causes the carbonation is slightly acidic. The carbon dioxide would be in the soft drink along with the phosphoric acid if it wasn’t decarbonated, which would cause the titration of phosphoric acid to be inaccurate. 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”.
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Unformatted text preview: Using the graph, calculate the apparent concentra±on of this tripro±c acid and the dissocia±on constants for all proton dissocia±on steps (K a1 , K a2 , and K a3 ). Approxima±ons of points on the graph 1 st equivalence point (12.5 ml, 4.9 pH) 2 nd equivalence 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 = 3.62 pKa2 = 6.2 pKa3 = 9.5 K a1 = 10 3.62 = 2.40 10 4 K a2 = 10 6.2 = 6.3 10 7 K a3 = 10 9.5 = 3.2 10 10 It took 12.5 mL to reach ³rst 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 NaOH Moles of tripro±c acid = moles of NaOH Therefore there are 0.00102 mol tripro±c acid 0.00102 mol tripro±c acid/0.07500 L tripro±c acid = 0.0136 M tripro±c acid The concentra±on of this tripro±c acid is 0.0136 M....
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