If 50 ml of 0.2 M KOH is added to 40 ml of 0.5 M HCOOH, the pH of the resulting solution is Ka = 1.8 × 10-4

pH=-log   Ka  +  log  SaltAcid Salt = 0.2 × 501000 = 0.01 ;   Acid = 0.5 × 401000 = 0.02 pH=-log 1.8 × 10-4 + log  0.010.02 pH=4-log 1.8 + log 0.5 pH = 4-log 1.8 - 0.301 pH = 3.4