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Question

Kb for a monoacidic base whose 0.2 M solution has a pH of 9.50 is:

A
5×108
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B
5×109
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C
6×106
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D
6×109
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Solution

The correct option is B 5×109
BOH B+ + OH
C 0 0
C(1α) Cα Cα

Given that, pH=9.5
pOH=149.5=4.5
We know,
pOH=log[OH]=4.5
[OH]=104.5

now, [OH]=C.α
(as α=Kb/C )
so, [OH]=Kb.C

Squaring both sides:
([OH])2=Kb.C104.5×2=Kb×0.2Kb=1090.2=5×109


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