Chem33-1, C-hour Problem Set 5

1.Derive an equation analogous to the Henderson-Hasselbalch equation that relates pOH and pKb of a buffered solution composed of a weak base and its conjugate aid, such as NH3 and NH4+. ANSWER: + −+ - [NH 4 ][OH ] NH 3 + H 2 O ⇔ NH 4 + OH K b = ; Taking the − log of the K b expression : [NH 3 ] - [NH 4+ ] − [NH 4+ ] − log Kb = − log [OH ] - log , − log[OH ] = − log K b + log [NH 3 ] [NH3 ] [NH 4+ ] [Acid] pOH = pKb + log or pOH = pKb + log [NH ] [Base] 32. 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 Ka 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.