18. A compound has a pKa of 7.4. To 100 mL of a 1.0 M solution of this compound at pH 8.0 is added 30 mL of 1.0 M hydrochloric acid. The pH value of resulting solution is:
(A) 7.5
(B) 7.4
(C) 7.2
(D) 6.8
(E) 6.5

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