4. A 0.02 M solution of monoprotic acid HA has a pH of 4.0. What are the values of the acid dissociation constant \(K_a\) and \(pK_a\)?
(A) \(K_a = 5.0 \times 10^{-7}\), \(pK_a = 6.30\)
(B) \(K_a = 5.0 \times 10^{-9}\), \(pK_a = 8.30\)
(C)\(K_a = 1.0 \times 10^{-4}\), \(pK_a = 4.00\)
(D)\(K_a = 2.0 \times 10^{-6}\), \(pK_a = 5.70\)
(E) \(K_a = 1.0 \times 10^{-5}\), \(pK_a = 5.00\)

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