緩衝溶液組合為過量弱酸+強鹼
NaOH和過量的CH3COOH反應完後會產生0.2mole的CH3COOH和0.3mole的CH3COONa
再用Henderson-Hasselbalch equation計算
PH=Pka+log[A-]/[HA]
PH=4.9反推得[H+]=1.2*10-5