Lect+23+08 - Lecture 23 Goals Examples for calculating...

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Lecture 23 Goals Examples for calculating concentrations and pH of weak acid solutions You will have a plain language understanding of buffers and the species that control buffering. Examples for calculating concentrations and pH of buffers and stressed buffer solutions You will have a conceptual understanding of Acid Base Titration curves and be able to figure out the primary species during titration Calculating the response of pH to titration You will have a plain language understanding of polyprotic acids and their neutralization
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Hydrolysis 4 + 3
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Weak Base Hydrolysis Prob. 10.38 The pH = 11.5 at 298 K of an aqueous solution of Na(CN) WHAT IS THE [CN - ] CONCENTRATION? Is HCN a weak Acid? CN - (aq) + H 2 O(l) HCN(aq) + OH - (aq) K a = 6.17 x 10 -10 and K a K b =10 -14 so K b = 1.6 x 10 -5 K b = [OH - ][HCN]/[CN - ] But [CN - ]= [OH - ] because weak base pOH = 2.50 since pH + pOH =14 and pH = 11.5 So [OH - ] = 10 -2.5 = 3.2 x 10 -3 = [HCN] [CN - ] = ([OH - ][HCN]/ K b )= 0.62M
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Buffers A buffer is a solution which maintains a relatively constant pH despite small additions of acid or base. HA(aq) + H 2 O(l) H 3 O + (aq) + A - (aq) If we add both HA and A - to the solution then there is chemical resistance to pH change when we add either acid or base. The equilibrium damps the effect of adding acid or base [ H 3 O + ]= K a ([HA]/[A - ]) and if both HA and A - are large then the [ H 3 O + ] is buffered Many physiological systems are buffered
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Calculations with buffers Calculations are similar to the equilibrium calculations in chapter 14 where both
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