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Chemistry Experiment 5

Chemistry Experiment 5 - Experiment 5 pH Study of...

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Experiment 5: pH Study of Phosphoric Acid in Cola Drinks By Jennifer Knotts Lab Partners: Katlin Geldbach Chemistry 1312, Section F-1, February 22, 2007
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Introduction: The lab was preformed to provide a study of phosphoric acid concentration in cola drinks and provide experience in proper K a ’s calculations. The “pleasant tartness” found in cola drink is due to the phosphoric acid content it contains. Phosphoric acid also slows the growth of molds and bacteria, which would otherwise multiply rapidly in the sugary solution. Almost all of the acidity of soda pop comes from the concentration of phosphoric acid and not from the carbonic acid. This can be verified by measuring the pH of fresh and flat soda pop; there's very little difference. Ph titration of Weak Acids and polyprotic acids differs slightly mainly because a weak acid only has one ionizable protein whereas a polyprotic acid has more than one ionizable proton per molecule. The equilibrium expressions for each of the two types of acids are shown below. Weak Acid equilibrium: HA (aq) + H 2 O H 3 O + + A - (aq) 1 st equivalence pt: [ ][ ] [ ] HA A O H K a - + = 3 Polyprotic Acid equilibrium: (H 3 PO 4 ) two equivalence points H 3 PO 4 + H 2 O ↔ H 3 O + + H 2 PO 4 _ K a1 = 7.5 x 10 -3 H 2 PO 4 - + H 2 O ↔ H 3 O + + HPO 4 2- K a2 = 6.2 x 10 -8 HPO 4 2- + H 2 O ↔ H 3 O + + PO 4 3- K a3 = 2.2 x 10 -13 1 st equivalence pt: [ ] 2 1 3 2 a a b w a K K K K K H = = + 2 nd equivalence pt: [ ] 2 3 2 3 a a b w a K K K K K H = = +
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