Calculate the corresponding value of Ka for formic acid

Question

A solution made by mixing 60.0 ml of 0.80 M formic acid, HCOOH, and 40.0 ml of 1.50 M sodium formate, NaHCOO, has a pH of 3.84. Calculate the corresponding value of Ka for formic acid

Answer 1

find molarities after the dilution caused by mixing:

0.80M @ 60.0ml / 100ml = 0.48 Molar acid

1.50M @ 40.0 ml / 100 ml = 0.60 Molar salt

pH = 3.84 ... gives a H+ = 1.45e-4

HCOOH --> H+  & COOH-

Ka = [1.45e-4] [0.60] / [0.48]

your answer : Ka = 1.8e-4

Ka = [H+] [COOH-] / [HCOOH]