20. a 0.15 m aqueous solution of the weak acid ha at 25.0 °c has a ph of 5.35. the value of ka for ha is __________. a) 3.0 × 10−5 b) 1.8 × 10−5 c) 3.3 × 104 d) 1.3 × 10−10 e) 7.1 × 10−9