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)3.5−5.5 B)4.5−6.5 C)5.5−7.5 D)6.5−8.5 E)None of these choices are correct.

Question

Quizplus · Accepted Answer

Propyl red changes color as the pH changes from its acid color (yellow) to its base color (red). The pKa of propyl red is approximately 7.7. K_a is the acid form of the indicator, while K⁻⁶ is the base form. At the equivalence point, when the acid and base forms are present in equal amounts, the pH will be equal to the pKa of the indicator. At this point, half of the indicator will be in its acid form, and half will be in its base form, resulting in the characteristic color change. Using the Henderson-Hasselbalch equation, we can solve for the pH at which half of the indicator is in the acid form and half is in the base form: pH = pKa + log([base]/[acid]) pH = 7.7 + log(1/1) pH = 7.7 From this calculation, we can see that the indicator will change color at a pH close to 7.7. Since the indicator is yellow below this pH and red above it, the approximate pH range over which it changes color is 4.5-6.5. Therefore, the best choice is B.

The Indicator Propyl Red Has Ka = 3

The Indicator Propyl Red Has K_a = 3