Question

Calculate the concentration of formate in a 100mm solution of formic acid at ph 4.15. The pka for formic acid is 3.75

Answer 1

The molarity of formic acid is 100 mM or $100\times 10^{-3}M$. The dissociation reaction of formic acid is as follows:

$HCOOH\leftrightharpoons HCOO^{-}+H^{+}$

The expression for dissociation constant of the reaction will be:

$K_{a}=\frac{[HCOO^{-}][H^{+}]}{[HCOOH]}$

Rearranging,

$[HCOO^{-}]=\frac{K_{a}[HCOOH]}{[H^{+}]}$

Here, pH of solution is 4.15 thus, concentration of hydrogen ion will be:

$[H^{+}]=10^{-pH}=10^{-4.15}=7.08\times 10^{-5}M$

Similarly, $pK_{a}=3.75$ thus,

$[K_{a}=10^{-pK_{a}}=10^{-3.75}=1.78\times 10^{-4}M$

Putting the values,

$[HCOO^{-}]=\frac{(1.78\times 10^{-4}M)(100\times 10^{-3}M)}{(7.08\times 10^{-5}M}=0.2511 M$

Therefore, the concentration of formate will be 0.2511 M.