Respuesta :
The molarity of formic acid is 100 mM or [tex]100\times 10^{-3}M[/tex]. The dissociation reaction of formic acid is as follows:
[tex]HCOOH\leftrightharpoons HCOO^{-}+H^{+}[/tex]
The expression for dissociation constant of the reaction will be:
[tex]K_{a}=\frac{[HCOO^{-}][H^{+}]}{[HCOOH]}[/tex]
Rearranging,
[tex][HCOO^{-}]=\frac{K_{a}[HCOOH]}{[H^{+}]}[/tex]
Here, pH of solution is 4.15 thus, concentration of hydrogen ion will be:
[tex][H^{+}]=10^{-pH}=10^{-4.15}=7.08\times 10^{-5}M[/tex]
Similarly, [tex]pK_{a}=3.75[/tex] thus,
[tex][K_{a}=10^{-pK_{a}}=10^{-3.75}=1.78\times 10^{-4}M[/tex]
Putting the values,
[tex][HCOO^{-}]=\frac{(1.78\times 10^{-4}M)(100\times 10^{-3}M)}{(7.08\times 10^{-5}M}=0.2511 M[/tex]
Therefore, the concentration of formate will be 0.2511 M.
