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Unformatted text preview: marks] Answer this question using the systematic treatment of equilibrium, clearly stating any
approximations you make and then checking them. (a) The pH of a 0.10M solution of acetic acid is 2.88.
What is the pKa for acetic acid? (b) What is the pH of a 0.10M solution of sodium acetate (the salt of the
conjugate base of acetic acid)?
(a) A solution made from FHA=0.10M Charge balance:
Plugging into the Ka: You could always make the approximation that since this is an acidic solution
you to write the approximate and familiar allowing For this question, though, you already know [H+] from the pH, allowing you to directly calculate And then pKa=4.756.
(b) A solution made from FNaA=0.10M
A sodium salt is soluble (it does have a solubility limit but throughout this course we have been dealing
with dilute solutions well below that solubility limit):
The equilibria are
Note that this will have to be a basic solution due to the formation of OH-.
Mass balance: , Equilibrium constant Substituting into the charge balance and rearranging: Substituting this into the first mass balance:
We can substitute this directly in the Kb expression Which we could solve through a lot of work, or we could recognize that in the basic solution that will
result from the hydrolysis of water,
and so we end with the familiar result yielding and and a pH of 8.88. 5. [50 marks] (a)...
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- Fall '11