Calculate Kb of a base

Question

A 0.660 M solution of a weak base (B:) is made. The solution has a pH of 9.19. Calculate the Kb of this base. Report your answer in scientific notation with 3 sig figs.

 

How do I find the Kb value given pH and the M of the solution?

Answer 1

The Kb = [BH+][OH-]/[B].

We know the concentration of the base [B] = 0.66 M, but since it's a weak base, we don't know how much OH- and BH+ exist in the solution. Since the base thats dissociates to form 1 OH- and 1BH+, the concentrations of them should be equal. So if we can determine [OH-], we will be able to solve the equation for Kb.

If the pH is 9.19 and pH + pOH = 14, then pOH = 5.81.

Knowing the pOH, we can calculate [OH-] = 10^-pOH = 10^-5.81 = 1.55 x 10^-6 M

We know [B] from the start of the problem, we just calculated [OH-], and we know that [BH+] should be the same, we can calculate Kb

Kb = (1.55 x 10^-6 M)(1.55 x 10^-6 M)/0.66 = 3.635 X 10^-12