Final answer:
To determine the Kb for the weak base with a pH of 11.28, first convert the pH to pOH. Then find the [OH-] concentration using the pOH value. Finally, calculate Kb using the equation Kb = [OH-]^2 / [B]. The Kb value for the base is 0.0024 M.
Step-by-step explanation:
To determine the Kb for the weak base, we need to first convert the pH to a pOH value. The pOH can be found by subtracting the pH from 14. In this case, the pOH is 14 - 11.28 = 2.72. The next step is to convert the pOH to [OH-] concentration by taking the antilog of the pOH value. [OH-] = 10^-pOH = 10^-2.72 = 0.0024 M.
Since Kb = [OH-]^2 / [B], where [B] is the initial concentration of the weak base, we need to find the concentration of the base. To do this, we use the equation for the dissociation of the base: B + H2O ⇌ BH+ + OH-. Since the base is weak, we can assume that the amount of BH+ formed is negligible compared to [B] and neglect it in our calculations. So, we can consider the amount of [B] to be equal to the amount of OH-. Therefore, [B] = 0.0024 M.
Now we can calculate Kb using the equation Kb = [OH-]^2 / [B]. Plugging in the values, we get Kb = (0.0024)^2 / 0.0024 = 0.0024 M.