The relationship between Ka and Kb for a conjugate acid-base pair is given by the equation:

Kb = Kw / Ka

where Kw is the ion product constant for water (1.0 x 10^-14 at 25 degrees Celsius).

Using the given Ka value, we can calculate the Kb for the conjugate base:

Kb = (1.0 x 10^-14) / (2.5 x 10^-4) = 4.0 x 10^-11

Therefore, the Kb for the conjugate base of the acid with Ka = 2.5 x 10^-4 at 25 degrees Celsius is 4.0 x 10^-11.