Lec-23-Chap16-4-bare

Lec-23-Chap16-4-bare - Lec-23-Chap16-4-bare 10/26/2011...

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Lec-23-Chap16-4-bare 10/26/2011 Buffer Solutions. Weak acid/Salt of conjugate base. 1 Lec-23: Buffer Solutions Chapter 16 1 Buffer Solutions As the name suggests, buffer solutions have the ability to withstand the addition of some amount of acid or base without changing their original pH by a large amount. Buffer solutions are made of approximately equal amounts of: weak acid and its conjugate base (HCH 3 COO/NaCH 3 COO) weak base and its conjugate acid (NH 3 /NH 4 NO 3 ) To see how the buffering mechanism works, consider a buffer solution made of acetic acid and sodium acetate. Such system will have a particular pH, and present in solution are the species : ; HAcO un Ac dissocia O con ted a jugat cid e : base 2 3 " We AcO use the notation f "CH " or COO"
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Lec-23-Chap16-4-bare 10/26/2011 Buffer Solutions. Weak acid/Salt of conjugate base. 2 Buffer Solutions If we add a certain amount of strong acid the acetate ion in solution will combine with the extra hydronium ions attenuating the otherwise large change in pH:         2 3 AcO aq H O aq HAcO aq HO  Conversely, if to the original solution we add a certain amount of base, the undissociated acetic acid will neutralize the added hydroxide ions, and thus attenuate the change in pH:         2 OH aq a H q O  We now turn to the quantitative aspects of buffer solutions. 3 : ; HAcO un Ac dissocia O con ted a jugat cid e : base Weak acid and its conjugate base: a buffer solution of this type is prepared by dissolving n A moles of the weak acid HA and n S moles of a salt of the conjugate base (for example NaA) in a volume V of solution. We want to calculate the pH of such buffer solution. We use the notations:         A A 0 S B 0 HA initial concentration of weak aci NaA initial concentration of conjugate ba d se n C V n C V  4 These concentrations explain how the solution was prepared, and they are referred to as analytical concentrations . Buffer Solutions
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Lec-23-Chap16-4-bare 10/26/2011 Buffer Solutions. Weak acid/Salt of conjugate base. 3             3 A A B B 2 A aq i HO HA aq H O a ni q e 0 t. q. C x C Cx Considering the equilibrium constant equation of the acid, we have   3 concentrations of the species at equi A l H ibri HA m O u a K   5     B A a x K It follows that NaA A s / p e bu ci ffer es i , n sol HA ution Buffer Solutions This equation gives an accurate relation for the concentration of hydronium ions in the buffer solution. It can be simplified under certain conditions.
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Lec-23-Chap16-4-bare - Lec-23-Chap16-4-bare 10/26/2011...

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