Protonation State We can estimate the fraction of each species present for a given acid or base for a given pH. Here pH is being controlled by the action of another chemical (see Ch. 8) Example 1: Acetic acid at pH’s 2, 4.7, 7, and the methylamonium ion at pH’s 7, 10, 13. Example 2: The methylamonium ion at pH’s 7, 10, 13. Polyprotic Acids The calculation of equilibria for polyprotic acids is done in a stepwisefashion. There is an ionization constant for each step. –Consider arsenic acid, H3AsO4, which has three ionization constants. Ka1 = 2.5 x 10-4 Ka2 = 5.6 x 10-8Ka3 = 3.0 x 10-13 The first ionization step for arsenic acid is: Second step: 443421-4243105.2AsOHAsOHHKaAsOHHAsOH−−++×==←+→8-14224a2-24-142106.5AsOHHAsOHKHAsOHAsOH−−++×==←+→Third step: 13-2434a3-34-24100.3HAsOAsOHKAsOHHAsO−−++×==←+→For weak polyprotic acids, Ka1is always bigger than Ka2, etc.
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