Ka and H+ TBR Gen Chem CHP 5 Passage 7 question 45

[katezilla2]

Confused on an answer from TBR Gen Chem CHP 5 Passage 7 question 45

What is true in the titration of acetic acid by sodium hydroxide when the pH of the solution is greater than the pKa for acetic acid?

I know right away the solution will be more basic so could eliminate C and D.

This leaves me with:
A. [H3CCO2-] > [H3CCo2H']; Ka (acetic acid) > [H+]
B. [H3CCO2-] > [H3CCo2H']; Ka (acetic acid) < [H+]

I picked B because I thought the Ka gets higher for something more acidic. When I went over it I realized that doesn't make sense because more H+ is more acidic. I don't understand the relationship between Ka and H+ here.

Thanks guys!

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[NextStepTutor_3]

This problem gets much easier if we first think of pH and pKa. At the half-equivalence point, [H3CCO2-] = [H3CCOOH] and pH = pKa. In the situation described by the question, we now have more conjugate base than acid, so we're past the half-equivalence point. pKa doesn't change, so now pH must be greater because the solution has become more basic. If pH is greater than pKa, then [H+] is less than Ka.

Your reasoning made sense - the Ka value is higher for a stronger acid, but we're only talking about acetic acid here, so Ka and pKa won't change. The only thing that will change is H+ concentration, and it's less because the solution is basic.

One other helpful thing: remember that since pH = pKa at the half-equivalence point, [H+] also equals Ka! It's a more uncommon way to describe their relationship but still true.

 

[katezilla2]

Thanks!