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Question

Consider a weak monobasic acid HA having a concentration C with degree of dissociation, α<<1.. The pH of this weak acid in terms of acid dissociation constant Ka and concentration C is :

A
log10(KaC)
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B
log10Ka.C
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C
pH=log10Ka.C2
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D
pH=log10K2a.C
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Solution

The correct option is B log10Ka.C
For weak acid HA with concentration C, ionization constant Ka and degree of dissociation (α<<1)
In aqueous solution,
HA(aq)H+(aq)+A(aq)
At t=0 C 0 0
Equilibrium CCα Cα Cα
Ka=Cα21α
As α<<1,(1α)1

Ka=Cα2

As H+ concentration is Cα
By multiplying C on both sides, we get:
Ka×C=Cα2×C=Ka×C=(Cα)2Ka×C=[H+]2
[H+]=Ka.C
pH=log10Ka.C

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