How do you do this indicator problem?

Question

Why is phenol red (pKaIn= 7.9) is most suitable for the titration of 0.10 M lactic acid with 0.10 M KOH (aq)? lactic acid pKa= 3.08

can you explain how this is done?

Answer 1

The best indicator for a titration is one whose transition range pH has a magnitude in the same bulk as that of the pH of the titration at the equivalence point.

Not enough info (vol acid and base; indicator transiton also helpful) to do the calculation but here's what to do:

(1) Determine the pH at the equivalence point (Kb = x^2/(F'-x); lactic acid is used up at this point; F' = [lactate  ion] derived [lactic acid] by dilution; x = [OH-] so change to [H3O+] using 10^-14 = [OH-][H3O+]

(2) Compare pH to transition pH for phenol red = 6.4 - 8.0 (DC Harris); answer in (1) should be in this range