Question

What is the ratio [ A − ] / [ HA ] [A−]/[HA] at pH 2.75 ? 2.75? The p K a pKa of formic acid (methanoic acid, H − COOH ) H−COOH) is 3.75.

Answer 1

Answer: The ratio of
`([A^-])/([HA])` is 0.1

Step-by-step explanation:

The chemical equation for the dissociation of monoprotic weak acid (formic acid) follows:

`HA\rightleftharpoons H^++A^-`

To calculate the pH of acidic buffer, we use the equation given by Henderson Hasselbalch:

`pH=pK_a+\log(([salt])/([acid]))`

`pH=pK_a+\log(([A^-])/([HA]))`

We are given:

`pK_a` = negative logarithm of acid dissociation constant of methanoic acid = 3.75

`([A^-])/([HA])` = ratio of concentration of salt and acid

pH = 2.75

Putting values in above equation, we get:

`2.75=3.75+\log(([A^-])/([HA]))\\\\([A^-])/([HA])=0.1`

Hence, the ratio of
`([A^-])/([HA])` is 0.1