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Question

Consider a weak monoacidic base BOH having a concentration C with dissociation α<<1. The pH of this weak base in terms of base dissociation constant Kb and concentration C is :

A
14+log10(KbC)
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B
14+log10Kb.C
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C
14log10Kb.C
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D
14log10(KbC)
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Solution

The correct option is B 14+log10Kb.C
For weak base BOH with concentration C, ionization constant Kb and α<<1.
In aqueous solution,
BOH(aq)B+(aq)+OH(aq)
At t=0 C 0 0
At t=t(eq) CCα Cα Cα

Kb=Cα21α

As α<<1(1α)1

Kb=Cα2
Multiplying both sides by C
Kb×C=Cα2×CKb×C=[OH]2
[OH]=Kb.C
pOH=log10Kb.CpH+pOH=14pH=14+log10Kb.C


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