During the titration of a weak monoprotic acid with a strong base
CHEMISTRY · Single correct · AP · Chemistry · Acids and Bases · Acids and Bases
Problem
During the titration of a weak monoprotic acid with a strong base, a student selects phenolphthalein rather than methyl orange to signal the endpoint. Which of the following best explains why phenolphthalein is preferred in this titration?
(a) Phenolphthalein has a pKa value closer to that of the weak acid being titrated, so its color change occurs at the half-neutralization point.
(b) Phenolphthalein undergoes a faster, more distinct color change than methyl orange, minimizing endpoint error.
(c) Phenolphthalein has the higher pKa, placing its transition range near the basic pH expected at the equivalence point of a weak-acid/strong-base titration.
(d) The very small Ka of phenolphthalein produces a sharper endpoint than methyl orange.