Posted by Meenaakshi on Friday, March 23, 2012 at 12:28am.
Given acidity constant,Ka of benzoic acid is 6.28 X 10^5. pH of 0.15 molar solution of this acid is ?
Here is my method:
Benzoic acid is a weak acid,hence it dissociates very little. So:
C6H5COOH> C6H5COO + H+
[H+] and [C6H5COO] are yet to be calculated,so let them be x
[C6H5COOH] = 0.15x
Since Ka= 6.28 X 10^5 mol/L,using the Kc equation:
Ka = x.x/(0.15x)
Assuming x<< 0.15,simplifying the eqn becomes : Ka = x.x/(0.15)
Solving the equation,x value is 3.069201851 X 10^3 mol/L = [H+].
pH = log[H+] = 2.51. Is this correct?
Thank you

Chemistry  DrBob222, Friday, March 23, 2012 at 12:31am
I agree. Thanks for showing your work. That helps a lot.