An aqueous solution of an unknown, monobasic weak base has an initial molarity of .176 M and a final[OH-] of .0000158 M. What is the Kb of the unknown base?

NH3 + HOH ==> NH4^+ + OH^-

Kb = (NH4^+)(OH^-)/(NH3)
Isn't this just a matter of substituting into Kb expression and solving for Kb?