2046chapter17

2046chapter17 - Chapter 17: Additional Aspects of Aqueous...

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1 Learning goals and key skills: h Describe the common ion effect. h Explain how a buffer functions. h Calculate the pH of a buffer solution. h Calculate the pH of a buffer after the addition of smal amounts of a strong acid or a strong base. h Calculate the pH at any point in an acid-base titration of a strong acid and strong base. h Calculate the pH at any point in a titration of a weak acid with a strong base or a weak base with a strong acid. h Understand the differences between titration curves for a strong-acid-strong base titration and those when either the acid or base is weak. h Calculate K sp from molar solubility and molar solubility from K sp . h Calculate the molar solubility in the presence of a common ion. h Predict the effect of pH on solubility. h Predict whether a precipitate wil form when solutions are mixed by comparing Q and K sp . h Calculate the ion concentrations required to begin precipitation. h Explain the effect of complex ion formation on solubility. Chapter 17: Additional Aspects of Aqueous equilibria Common-ion effect The extent of ionization of a weak electrolyte is decreased by adding to the solution a strong electrolyte that has an ion in common with the weak electrolyte. Note: this is merely an application of Le Châtelier’s principle. CH 3 COOH ( aq ) + H 2 O ( l ) H 3 O + ( aq ) + CH 3 COO - ( aq) Common-ion effect HA (aq) + H 2 O (l) A - (aq) + H 3 O + (aq) Having a common ion (the conjugate base, A - ) will limit the ionization of the weak acid (HA). B (aq) + H 2 O (l) BH + (aq) + OH - (aq) Similarly, having a common ion (the conjugate acid, BH + ) will limit the ionization of the weak base (B).
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2 The Common-Ion Effect Calculate the fluoride ion concentration and percent ionization of solutions that are: a) 0.200 M in HF b) 0.200 M in HF and 0.100 M in HCl. The K a for HF is 6.8 × 10 ±4 . [H 3 O + ] [F - ] [HF] K a = = 6.80 × 10 -4 HF ( aq ) + H 2 O ( l ) H 3 O + ( aq ) + F - ( aq ) [HF], M [H 3 O + ], M [F ± ], M Initial Change Equilibrium Example: common-ion effect A 0.10 M lactic acid solution (K a = 1.4 × 10 -4 ) has a pH of 2.43 and 3.7% ionization of the acid. Let’s calculate the pH and percent ionization of a 0.10 M lactic acid solution when 0.10 M sodium lactate is added. lactic acid (aq) + H 2 O (l) lactate ion (aq) + H 3 O + (aq) HLac(aq) + H 2 O (l) Lac - (aq) + H 3 O + (aq) (alternative equation) Buffers: solutions that resist changes in pH upon addition of small amounts of acids/bases. • Usually prepared from solutions of a weak conjugate acid- base pair. • They are particularly resistant to pH changes, even when strong acid or base is added.
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3 Henderson-Hasselbalch equation Use only with valid assumptions! pH = p
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This note was uploaded on 12/13/2011 for the course CHM 2046h taught by Professor Lufaso during the Fall '11 term at UNF.

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2046chapter17 - Chapter 17: Additional Aspects of Aqueous...

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