## Chemistry: The Molecular Science (5th Edition)

Methyl red is the most suitable, because it changes its color in acidic solutions, and the equivalence point of the titration between $NH_3$ and $HBr$ is in acidic solution.
$NH_3$ is a weak base and $HBr$ is a strong acid; therefore, the pH at the equivalence point of their titration will be acidic $(pH < 7)$. Analyzing the 3 given indicators on page 667, the one where the color changes in acidic solution is the most appropriate, so Methyl red is the correct answer.