Posted by jack on Wednesday, November 24, 2010 at 12:44pm.
The pH of a solution that contains 0.818 M acetic acid (Ka = 1.76 x 105) and 0.172 M sodium acetate is
__________. The Ka of acetic acid is 1.76 × 105.

chemistry  Dr Russ, Wednesday, November 24, 2010 at 1:04pm
You are missing units for your Ka.
I am sure that I have answered this one recently. Anyway start from the equation
HAc <> H+ + Ac
so
Ka = [H+][Ac]/[HAc]
at the start we have 0.181 M HAc
and 0.172 M NaAc
at equilibrium we have
(0.181x)M HAc (some has dissociated)
(0.172+x)M Ac (we now have more Ac due to the dissociation)
x M H+
so we can write Ka
(0.172x)(x)/(0.181x)= 1.76 × 105
you can either solve the quadratic or we can say that x is small wrt 0.172 and 0.181, hence
(0.172)(x)/(0.181)= 1.76 × 105
find x
hence find the pH=log(x)

chemistry  jack, Wednesday, November 24, 2010 at 1:04pm
sorry, answer found. no longer need help..

chemistry  Dr Russ, Wednesday, November 24, 2010 at 1:10pm
I hope the method above agrees with the answer you have found.
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