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Question

The percentage degree of dissociation of 0.05 M acetic acid (CH3COOH) solution with Ka=1.8×105 is :

A
0.55
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B
5.5
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C
1.9
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D
19
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Solution

The correct option is C 1.9
Ka=1.8×105, degree of dissociation (α)
The dissociation of acetic acid is given by
CH3COOH (aq.)H+ (aq.)+CH3COO (aq.)Initial: C 0 0At equilibrium: C(1α) Cα Cα

Ka=[CH3COO][H+][CH3COOH]=(Cα)2C(1α)Ka=Cα21α1.8×105=0.05×α21α
Since CH3COOH is a weak acid,
So, α<<11α1

1.8×105=0.05×α2α=1.8×1050.05=3.6×104α=1.9×102=0.019
% α=0.019×100=1.9%

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