`pH = 2,72 => h = [H^+] = 10^{-2,72}`

`=> [OH^-] = 10^{-11,28} (M)`

`[CH_3COO^-] = (K_{a,CH_3COOH})/(K_{a,CH_3COOH} + h) . C_{CH_3COOH}^o = 9,36.10^{-4} (M)`

`[HCOO^-] = (K_{a,HCOOH})/(K_{a,HCOOH} + h) . C_{HCOOH}^o = 0,0863x (M)`

Ta có các cân bằng:

$CH_3COOH \rightleftharpoons CH_3COO^- + H^+$

$HCOOH \rightleftharpoons HCOO^- +H^+$

$H_2O \rightleftharpoons H^+ + OH^-$

Áp dụng ĐKP:

`[H^+] = [CH_3COO^-] + [HCOO^-] + [OH^-]`

`=> 10^{-2,72} = 9,36.10^{-4} + 0,0863x + 10^{-11,28}`

`=> x ~~ 1,12.10^{-2} M`