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Unit 6 - Chpt 15 - Acid/Base Equilibria

Unit 6 - Chpt 15 - Acid/Base Equilibria. Common Ion Effect Buffers / Buffer Capacity Titration / pH curves Acid / Base Indicators HW set1: Chpt 15 - pg. 736-742 # 17, 19, 21, 23, 25, 34, 38, 40, 44 Use Appendix 5 for K a K b values - Due Mon. Mar 3. Common Ion Effect.

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Unit 6 - Chpt 15 - Acid/Base Equilibria

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  1. Unit 6 - Chpt 15 - Acid/Base Equilibria • Common Ion Effect • Buffers / Buffer Capacity • Titration / pH curves • Acid / Base Indicators • HW set1: Chpt 15 - pg. 736-742 # 17, 19, 21, 23, 25, 34, 38, 40, 44 Use Appendix 5 for Ka Kb values - Due Mon. Mar 3

  2. Common Ion Effect • Shift in equilibrium position that occurs because of the addition of an ion already involved in the equilibrium reaction. • An application of Le Châtelier’s principle. HCN(aq) + H2O(l) H3O+(aq) + CN-(aq) • Addition of NaCN will shift the equilibrium to the left because of the addition of CN-, which is already involved in the equilibrium reaction. • A solution of HCN and NaCN is less acidic than a solution of HCN alone.

  3. Ka Problem with common ion Calculate the pH of a 0.50 M aqueous solution of the weak acid HF (Ka = 7.2 x 10–4) and 0.10 M NaF (a strong electrolyte). HF(aq) + H2O H3O+(aq) + F–(aq) Initial 0.50 M ~ 0 0.10M Change –x +x +x Equilibrium 0.50–x x 0.10M + x [H3O+]= 3.6 x 10–3 pH = 2.44 What was pH before adding NaF?

  4. Buffered Solutions • Buffered Solution – resists a change in pH. • They are weak acids or bases containing a common ion. • After addition of strong acid or base, deal with stoichiometry first, then the equilibrium.

  5. Buffer Problems flow chart

  6. Buffers - How do they work?

  7. Buffers - How do they work? (2)

  8. Henderson–Hasselbalch Equation • For a particular buffering system (conjugate acid–base pair), all solutions that have the same ratio [A–] / [HA] will have the same pH.

  9. Buffer Problem What is the pH of a buffer solution that is 0.45 M acetic acid (HC2H3O2) and 0.85 M sodium acetate (NaC2H3O2)? The Ka for acetic acid is 1.8 × 10–5. pH = 5.02

  10. Buffers graphic

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