06Fexam1key - Name_KEY EID CH 339K Exam 1 7 Sept 2006 100...

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Name______________ KEY _____________________________ EID__________________________________________ 1 CH 339K Exam 1 7 Sept 2006 100 points total 7 pts. 1. What is the pH of a solution of 10 -10 M HCl in water? 7.0 (the contribution of protons from the complete dissociation of 10 -10 M HCl is insignificant compared to the 10 -7 M H + already present in pure water) 25 pts. 2 . a . An enzymatic reaction takes place in a 5-ml solution that has a total citrate concentration of 200 mM and an initial pH of 3.5. During the reaction, 0.15 mmols of acid are produced. Using the pK a values below, calculate the final pH of the solution. (Hint: first calculate the initial ratio of [A - ]/[HA], before the reaction begins) citrate <==> H + + citrate -1 pK a = 3.75 citrate -1 <==> H + + citrate -2 pK a = 4.76 citrate -2 <==> H + + citrate -3 pK a = 6.40 At initial pH of 3.5, only citrate and citrate -1 will be present in significant amounts, so use pK a = 3.75: HA A - citrate H + + citrate -1 Initial: 200 0 0 Final: 200 – x x x pH = pK a + log [ A " ] [ HA ] 3.5 = 3.75 + log x 200 " x " 0.25 = log x 200 " x 10 " 0.25 = x 200 " x 0.562 = x 200 " x 112.5 " 0.562 x = x 112.5 = 1.562 x x = 72 mM = [ citrate " 1 ] [ citrate ] = 200 " x = 200 " 72 = 128 M Now, the reaction adds 0.15 mmols of acid (0.15 x 10 -3
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