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 10-5) and 0.172 M sodium acetate is

__________. The Ka of acetic acid is 1.76 × 10-5.

- 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

(0.181-x)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

(0.172-x)(x)/(0.181-x)= 1.76 × 10-5

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 × 10-5

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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