The correct option is B 5×10−9
BOH ⇌ B+ + OH−
C 0 0
C(1−α) Cα Cα
Given that, pH=9.5
⇒pOH=14−9.5=4.5
We know,
pOH=−log[OH−]=4.5
[OH−]=10−4.5
now, [OH]−=C.α
(as α=√Kb/C )
so, [OH]−=√Kb.C
Squaring both sides:
([OH−])2=Kb.C10−4.5×2=Kb×0.2Kb=10−90.2=5×10−9