A 0.10 M solution of a weak acid HA has a pH of 3.00. What is the acid dissociation constant, Ka, for HA?
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Correct answer
A. 1.0 x 10^-5
Principle or equation
For a weak monoprotic acid, [H+] = sqrt(Ka * C) when ionization is small. Ka = [H+]^2 / C.
Why this answer is correct
Given pH = 3.00, [H+] = 1.0 x 10^-3 M. For a weak acid, Ka ≈ [H+]^2 / C = (1.0 x 10^-3)^2 / 0.10 = 1.0 x 10^-5.
Example
If a 0.20 M weak acid has pH 2.70, [H+] = 2.0 x 10^-3 M, Ka ≈ (2.0 x 10^-3)^2 / 0.20 = 2.0 x 10^-5.
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