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* 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)=([HCOO^(-)][H^(+)])/([HCOOH])`

Rearranging,

`[HCOO^(-)]=(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* 10^(-5)M`

Similarly,
`pK_(a)=3.75` thus,

`[K_(a)=10^{-pK_(a)}=10^(-3.75)=1.78* 10^(-4)M`

Putting the values,

`[HCOO^(-)]=((1.78* 10^(-4)M)(100* 10^(-3)M))/((7.08* 10^(-5)M)=0.2511 M`

Therefore, the concentration of formate will be 0.2511 M.