2015 AP Chemistry free response 3c

- [Voiceover] The pH at the equivalence point of the titration is measured to be 2.54. Which of the following indicators would be the best choice for determining the end point of the titration? Justify your answer. Well let's just remind ourselves, the endpoint of the titration is when the indicator changes color, and because that's our signal, our visual signal, it's saying, "Hey, we must've hit the equivalence point," you want the pKa of your indicator to be as close as possible to the pH at the equivalence point. If the pH at the equivalence point is 2.54, well which of these indicators have the closes pKa to that? Well we see thymol blue has it at two right over here, so we would choose to use thymol blue, thymol blue, because its pKa closest to pH at equivalence point. Equivalence, equivalence point. There you go.