The Acid Dissociation Constant, K a. The Acid Dissociation Constant. When a weak acid dissolves in water, it does not completely dissociate.
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HA(aq) + H2O(l) H3O+(aq) + A-(aq)
Kc = [H3O+][A-]
Kc[H2O] = Ka = [H3O+][A-]
Use the equation for the dissociation of propanoic acid in water to set up an ICE table.
CH3CH2COOH(aq) + H2O(l) CH3CH2COO-(aq) + H3O+
Initial 0.10 mol/L 0 0
Change -x +x +x
Equil 0.10 - x x x
Ka = [CH3CH2COO-][H3O+]
(0.10 - x)
The value of x is equal to [H3O+] and [CH3CH2COO-].
[H3O+] = 10-2.96
= 1.1 x 10-3 mol/L
Ka = (1.1 x 10-3)2
0.10 - (1.1 x 10-3)
= 1.2 x 10-5
Percent dissociation = 1.1 x 10-3 mol/L x 100
= 1.1 %