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2046chapter17

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

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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.
3 Henderson-Hasselbalch equation Use only with valid assumptions! pH = p K a + log [base] [acid] HA (aq) + H 2 O (l) A - (aq) + H 3 O + (aq) K a = [A - ] [H 3 O + ] [HA] = [A - ] [H 3 O + ] [HA] -log K a = -log [H 3 O + ] + -log [A - ] [HA] p K a pH acid conj. base What is the pH of a buffer that is 0.12 M in lactic acid, CH 3 CH(OH)COOH, and 0.10 M in sodium lactate? K a for lactic acid is 1.4 × 10 -4 .

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

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