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Unformatted text preview: otein behavior. 3. [30 marks] A solution containing acetic acid, oxalic acid, ammonia and pyridine has a pH of 9.00. All
relevant equilibria are shown below. You do not know any formal concentrations of any of the species.
(a) What fraction of ammonia is not protonated? (b) What is the principal species of oxalic acid in the
solution (prove your answer)?
Species
Acetic acid, HAc
Oxalic acid, H2Ox Equilibria Equilibrium constant Ammonia, NH3
Pyridine, RN
Regardless of how many equilibria are present, each individual equilibrium must be individually
satisfied. Therefore,
so that
is . The fraction of unprotonated ammonia where the denominator comes from the mass balance for ammonia. Multiplying the righthand side by 1, where yields where ratio is known. The fraction of unprotonated ammonia is0.363.
A more pedestrian route:
You could also rearrange to yield and then substitute into . (b) For the principal species of oxalic acid:
. Since and . . Since
Since ,
, and . , Ox2 is the principal species. 4.[70...
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 Fall '11
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