Fall 01. Exam3 key

Fall 01. Exam3 key - Chem 2101 Hour Exam II 2001 General...

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Unformatted text preview: Chem 2101 Hour Exam II 2001 General Note to TAs: If students write c B instead of c AH in the following equation [H + ] 1. appr. = K AH c AH ; [H + ] 2. appr. = K AH (c AH- [H + ] 1. appr. ) or just the opposite in [OH- ] 1. appr. = K b c B ; [OH- ] 2. appr. = K b (c B- [OH- ] 1. appr. ) or if they use equivalent expressions such as c tot , please accept this as fully correct as long as they subsequently plug in the correct number. {One may consider c AH as the formal concentration of the acid even after this acid has been titrated and converted to the base.} Name 1 (15 points) Design a 10 mM buffer solution to have a pH = 7.25. This requires selecting the most appropriate acid from the attached list, and specifying the formal concentrations of all species that you would add to the solution. You can assume that all activity coefficients are 1 but you must justify all approximations that you use. The most appropriate acid is dihydrogenphosphate because 10-7.25 = 5.6 10-8 is closest to any K value given in the supplied list. The choice of a less appropriate acid gives a deduction of 5 points. pH = pK a + log c HPO 4 2- + [H + ] - [OH- ] c H 2 PO 4- - [ H + ] + [OH- ] At pH 7.25 the [H + ] and [OH- ] concentrations are 10-7.25 and 10-6.75 , respectively, and are therefore much smaller than the buffer concentration. This allows the approximation: pH = pK a + log c HPO 4 2- c H 2 PO 4- 7.25 = log (6.3 10-8 ) + log c HPO 4 2- c H 2 PO 4- 7.25 = 7.20 + log c HPO 4 2- c H 2 PO 4- 0.05 = log c HPO 4 2- c H 2 PO 4- 1.12 = c HPO 4 2- c H 2 PO 4- => c HPO 4 2- = 1.12 c H 2 PO 4- c HPO 4 2- = 1.12 1 + 1.12 10 mM = 5.29 mM ; c H 2 PO 4- = 1 1 + 1.12 10 mM = 4.71 mM Name...
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This note was uploaded on 10/02/2011 for the course CHEM 2101 taught by Professor Bulman during the Spring '11 term at Minnesota.

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Fall 01. Exam3 key - Chem 2101 Hour Exam II 2001 General...

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