Chp16,Buffers - Chang 7th Edition Chapter 16 Worksheet#1 S...

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th Edition, Chapter 16, Worksheet #1 S. B. Piepho, Fall 2002 Buffers A buffer is a solution that resists changes in pH . The pH of a buffer changes very little when small amounts of a strong acid or strong base are added to the buffer. A buffer consists of approximately equal amounts of a conjugate weak acid/weak base pair in equilibrium with each other . Strong acids and their conjugate bases don’t produce a buffer since strong acid ionization is complete: there is no equilibrium! Acidic Buffers In an acidic buffer both the weak acid and conjugate base are present initially in roughly equal concentrations. The equilibrium is HA H + + A weak acid conjugate base For a weak acid, K a = [H + ][A ] [HA] so p K a = pH - log [A - ] [HA] . Thus pH = p K a + log [A - ] [HA] : this is the Henderson-Hasselbalch equation . These equations are useful for estimating the pH of a buffer. When [A - ] = [HA], it follows that K a = [H + ] and pH = pK a . Acidic Buffer Example: Acetic acid (HC 2 H 3 O 2 ) , sodium acetate ( NaC 2 H 3 O 2 ) NaC 2 H 3 O 2 is a soluble salt that dissolves in water to give Na + and C 2 H 3 O 2 - ions. Na + ions are neutral spectator ions, so can be ignored. The C 2 H 3 O 2 - ions provide the conjugate base for the buffer. The buffer equilibrium is HC 2 H 3 O 2 H + + C 2 H 3 O 2 acetic acid acetate ion For acetic acid, K a (HC 2 H 3 O 2 ) = 1.8 x 10 -5 so p K a = -log 10 K a = 4.74. Thus when [HC 2 H 3 O 2 ] = [C 2 H 3 O 2 ], pH = p K a = 4.74. Basic Buffers In a basic buffer both the weak base and conjugate acid are present initially in roughly equal concentrations. The equilibrium is B + H 2 O BH + + OH weak base conjugate acid For a weak base,
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Chp16,Buffers - Chang 7th Edition Chapter 16 Worksheet#1 S...

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