The indicator propyl red has K_a = 3.3 * 10¯⁶. What would be the approximate pH range over which it would change color? A) 3.5 to 5.5 B) 4.5 to 6.5 C) 5.5 to 7.5 D) 6.5 to 8.5 E) None of these choices is correct.

Question

Quizplus · Accepted Answer

The pH range over which an indicator changes color is typically around the pKa ± 1. The pKa of propyl red is approximately 5.5 (since pKa = -log(Ka) = -log(3.3 * 10¯⁶)). Therefore, the color change occurs approximately between pH 4.5 and 6.5.

The Indicator Propyl Red Has Ka = 3

The Indicator Propyl Red Has K_a = 3