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Consider the dissociation equilibrium of the following weak acid HA H+(aq) + A-(aq) If the pKa of the acid is 4, then the pH of 10 mM HA solution is . (Nearest integer) [Given: The degree of dissociation can be neglected with respect to unity]
3
Ka = 10-4, C = 10 mM = 10-2 M
α ≪ 1 Ka = C 2
[H+] = C = Ka C
= 10-4 10-2 = 10-3 M
pH = - (10-3) = 3
(Equivalently, pH = 12(pKa - C) = 12(4 + 2) = 3)
Answer: 3
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