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Unformatted text preview: M acetic acid we looked at earlier. Calculate the concentrations of the species in 0.15 M hydrocyanic acid, HCN, solution. K a = 4.0 x 10-10 for HCN We'll fill this out and compare them: 0.15 M HCN 0.15 M acetic acid % pH [H + ] K a Solution Le Chatelier's Principle applied to Dilution of Weak Acids [ ][ ] [ ] HA A O H K 3 a − + = Add enough water so that [ ][ ] [ ] 10 / K 10 HA / 10 / A 10 / O H Q a 3 = = − + all concentrations decrease by a factor of 10: System will move to the RIGHT: Acid will dissociate further! As Q increases back to be equal to K a , the % dissociation increases! K a vs. % Dissociation Although BOTH indicate if an acid is strong or weak: K a is CONSTANT for constant T no matter the dilution : % dissociation will INCREASE as dilution increases: See Fig 7.5...
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This note was uploaded on 06/13/2011 for the course CH 302 taught by Professor Holcombe during the Spring '07 term at University of Texas.
- Spring '07