81. The pH of a bottle of vinegar is 2.75 at 25℃. What is the mass percentage concentration (w/w) of acetic acid in this vinegar, assuming no other acid is presented? (Ka of acetic acid: 1.8x10-5； pKa of acetic acid: 4.74； 10-2.75 = 0.0018； 102.75 = 562.3； assuming the density of vinegar = 1 g/cm3 ； CH3COOH=60 g/mol)
(A) 5.0%
(B) 4.0%
(C) 3.0%
(D) 2.0%
(E) 1.0%
答案：E
統計：A:0,B:0,C:0,D:0,E:1
難度：計算中