Question

The pH of a 1.2 M solution of an unknown weak base is 10.65. What is the Kb for the base?

Answer 1

BOH ==> B^+ + OH^-

Kb = (B^+)(OH^-)/(BOH)
You know pH, convert to pOH, then to (OH^-) and substitute into Ka expression. For (BOH), substitute 1.2 - (OH^-) and solve for Kb.