Answer to Question #125341 in Inorganic Chemistry for zeeshan

Question

An indicator is a weak acid and pH range of its colour change is 3.1 to 4.5.if the neutral point of indicator in the center of this pH range,What is ionization constant of the indicator?

Answer 1

The ionization equilibrium of the indicator:

HA = H⁺ + A^-

ionization constant is: K = [H⁺]*[A^-]/[HA]

at neutral point: [A^-] = [HA],

hence, K = [H⁺]

if the neutral point is in the center of the pH range 3.1-4.5, it is at the pH = (4.5 + 3.1)/2 = 3.8

Thus, ionization constant is K = 10^-3.8 = 1.58*10^-4