Question

The indicator propyl red has a Ka of 3.3 X 10-6 . It is red at low pH and yellow at higher pH. What is the approximate pH range over which it changes color

Answer 1

Answer:

4.48 - 6.48

Explanation:

A pH indicator works in a better way in a range of pH = pKa ± 1. That means we need to determine the pKa of the indicator propyl red to find the range over which it change its color. That is:

pKa = -log Ka

pKa = -log 3.3x10⁻⁶

pKa = 5.48

That means the range over propyl red will change from yellow to red or vice versa is:

4.48 - 6.48