HCOOH ⇌ HCOO– + H+
\[\mathrm{pH} = \mathrm{p}K_\mathrm{a} - \log \left(\frac {c_\mathrm{syra}}{c_\mathrm{bas}} \right)\]
\[\begin{aligned}c_\mathrm{syra} &= \frac {c_\mathrm{HCOOH} \cdot V_\mathrm{HCOOH}}{V_\mathrm{tot}} = \frac {0,050\frac {\mathrm{mol}}{\mathrm{dm^3}} \cdot 0,075\mathrm{dm^3}}{(0,075 + 0,025)\mathrm{dm^3}} = 0,0375\mathrm{mol/dm^3} \\ c_\mathrm{bas} &= \frac {c_\mathrm{HCOO^-} \cdot V_\mathrm{HCOO^-}}{V_\mathrm{tot}} = \frac {0,0100\frac {\mathrm{mol}}{\mathrm{dm^3}} \cdot 0,025\mathrm{dm^3}}{(0,075 + 0,025)\mathrm{dm^3}} = 0,0025\mathrm{mol/dm^3}\end{aligned}\]
\[\mathrm{pH} = 3,77 - \log \left(\frac {0,0375\mathrm{mol/dm^3}}{0,0025\mathrm{mol/dm^3}} \right) = 2,59390874 ≈ 2,59\]
