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\times10^{-7}, pK{a}=6.30

(B) K{a}=5.0\times10^{-9}, pK{a}=8.30

(C) K{a}=1.0\times10^{-4}, pK{a}=4.00

(D) K{a}=2.0\times10^{-6}, pK{a}=5.70

(E) K{a}=1.0\times10^{-5}, pK{a}=5.00

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