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# probset5withanswers - Chem33-1 C-hour Problem Set 5 Student...

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Chem33-1, C-hour Problem Set 5 Due Date: Nov. 2, 2007 Student name: ___________________ Student SISD #: __________________ 1. Derive an equation analogous to the Henderson-Hasselbalch equation that relates pOH and pK b of a buffered solution composed of a weak base and its conjugate aid, such as NH 3 and NH 4 + . ANSWER: NH 3 + H 2 O NH 4 + + OH - K b = [NH 4 + ][OH ] [NH 3 ] ; Taking the log of the K b expression: log K b =− log [OH - ] - log [NH 4 + ] [NH 3 ] , log[OH ] = log K b + log [NH 4 + ] [NH 3 ] pOH = pK b + log [NH 4 + ] [NH 3 ] or pOH = pK b + log [Acid] [Base] 2. A sample of a certain monoprotic weak acid was dissolved in water and titrated with 0.125 M NaOH, requiring 16.00 mL to reach the equivalence point. During the titration, the pH after adding 2.00 mL of NaOH was 6.912. Calculate K a for the weak acid. ANSWER: At equivalence point: 16.00 mL x 0.125 mmol/mL = 2.00 mmol OH - added; There must be 2.00 mmol HX present initially.

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2.00 mL NaOH added = 2.00 mL x 0.125mmol/mL = 0.250 mmol OH - ; 0.250 mmol of OH - added will convert 0.250 mmol HX into 0.250 mmol X - ; Remaining HX = 2.00 – 0.250 = 1.75 mmol HX; This is a buffer solution where [H + ] = 10 -6.912 = 1.22 x 10 -7 M. Since total volume cancels: K a = [H + ] [X - ] [HX]
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probset5withanswers - Chem33-1 C-hour Problem Set 5 Student...

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