10 - Chem204 Lecture10 We can now consider a wider variety...

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Chem 204–Lecture 10 We can now consider a wider variety of aqueous equilibria. If you noticed last week, the neutralization of H 3 PO 4 had some interesting features Note the three regions with slow pH changes. http://www.bio.cmu.edu/courses/03231/LecF04/Lec03/PiTitrText.gif
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Lecture 10 –page 2 In each of these regions, a weak acid and its conjugate base were present. HA(aq) + H 2 O(l) H 3 O + (aq) + A - (aq); K a . K a =[H 3 O + ][A - ]/[HA]; K a [HA]/[A - ] = [H 3 O + ] So pH = pK a –log 10 ([HA]/[A - ]) With equal concentrations of [HA] and [A - ] pH = pK a . pH = 4.75 for acetic acid/acetate ion. K a =1.8 x 10 -5 . For a weaker acid/base pair, NH 4 + /NH 3 ,
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Lecture 10 –page 3 But here is where things get interesting. After starting with equal concentrations of acid/conjugate base, add strong acid until the ratio of [HA]/[A - ] = 2 pH = pK a –log 10 (2) = 4.75 – 0.3 = 4.45 Reverse and add strong base, [HA]/[A - ] = ½ pH = pK a –log 10 (1/2) = 4.75 + 0.3 = 5.05. We added considerable amounts of strong acid or
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Lecture 10 –page 4 To summarize our observations: pK a = 4.75 [HA]=[A-] Buffered region Small Δ pH
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Lecture 10 –Page 5 Solutions which resist substantial change in pH upon addition of acid/base are called Buffers.
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This note was uploaded on 11/19/2010 for the course CHEM 204 taught by Professor Gruebele during the Spring '08 term at University of Illinois, Urbana Champaign.

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10 - Chem204 Lecture10 We can now consider a wider variety...

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