posted by Martha .
A 0.513 M solution of a weak base has a pH of 11.4. What is the Kb of the base?
We will assume that this is monobasic and the temp is 25C so that Kw=10^-14
If pH is 11.4 then pOH is 14-11.4 = 2.6
so [OH-] = 10^-2.6 = 2.512 x 10^-3
assuming that [weak base] at equilibrium is large compared with [OH-] then
Kb is approximately [OH-]^2/[weak base]
Kb=(2.512 x 10^-3)^2/0.513
=1.23 x 10^-5
but check my maths.