Q. Fixed volume of 0.1 M benzoic acid (pKa = 4.2) solution is added into 0.2 M sodium benzoate solution and formed a 300 mL. resulting acidic buffer solution. If pH of the resulting solution is 3.9, then added volume of benzoic acid is
Answer = 100 mL
Putting the given values, 4.5=4.2+log[Salt][Acid]
[Salt][Acid] = antilog 0.3 = 2
Let v mL of benzoic acid is mixed with (300-v) mL of 1M sodium benzoate.
v = 100 mL