What is the ionisation constant of weak base ?

Question

What is the ionisation constant of a weak base that produces one OH- per molecule if there is 2.5% ionisation of .05M soln ?

Answer 1

Ok, lets call this weak base MOH where M+ is the cation and OH- is the anion.

MOH(aq)<===>OH-(aq) + M+(aq)

2.5% of 0.05 M=(2.5/100)*0.05 M=0.00125 M

Kb=[OH-][M+]/[MOH]

=[0.00125][0.00125]/[0.05-0.00125]

=(0.00125)²/(0.05-0.00125)

=3.2x10^-5