pKanotesCHEM216W11 - Chem 216 W11 Notes Topic: _pKa_ page 1...

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Chem 216 W11 Notes Topic : ___pK a __ page 1 of 2. Acidity Constant (pK a ) AH + H 2 O A - + H 3 O + K e K e = (a A - )(a H 3O + ) (a AH )(a H 2O ) (a X ) : activity of each species; K e : equilibrium constant At low concentrations, (a H 2 O ) constant. Therefore, K a = K e • (a H 2 O ) = (a A - )(a H 3O + ) (a AH ) = [A - ][H 3 O + ] [AH] where K a : (thermodynamic) acidity constant [X] : concentration of each species; f : activity coefficient, i.e., (a X ) = f X • [X] This equation can be approximated as follows in a dilute solution: K a [A - ][H 3 O + ] [AH] = K a K a = acidity constant pK a = - log K a thus, smaller pK a stronger acid pH vs. pK a Since pH = - log [H + ] = log 1 [H + ] , note: [H + ] = [H 3 O + ] K a = [A - ][H 3 O + ] [AH] can be transformed into pK a = log [AH] [A - ] + pH Note: The pH of a solution can be adjusted by adding an acid or base. For example, Acetic acid [CH 3 C(=O)OH] has a pK a of 4.76 At pH = 4.76, 4.76 = log [AH] [A - ] + 4.76 log
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This note was uploaded on 03/19/2011 for the course CHEM 216 taught by Professor Nolta during the Spring '07 term at University of Michigan.

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pKanotesCHEM216W11 - Chem 216 W11 Notes Topic: _pKa_ page 1...

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