Question

An unknown weak acid, HA, it titrated with 1.2 M NaOH. The pH at the halfway point of this titration was found to be 4.081. If the initial pH of the weak acid solution (before titration) has a pH of 2.348, what was the concentration of the weak acid solution

Answer 1

Answer: The initial concentration of weak acid solution is 0.243 M

Step-by-step explanation:

We are given:

pH of unknown acid at the halfway point of titration = 4.081

We know that:

`pH=pK_a=4.081` (At halfway point)

To calculate the
`K_a`, we use the equation:

`pK_a=-\log (K_a)`

Putting values in above equation, we get:

`4.081=-\log (K_a)\\\\K_a=10^(-4.081)=8.298* 10^(-5)`

As, the initial pH of the acid is 2.348

• To calculate the hydrogen ion concentration, we use the equation:

`pH=-\log [H^+]`

Putting values in above equation, we get:

`2.348=-\log [H^+]`

`[H^+]=10^(-2.348)=4.487* 10^(-3)`

The chemical equation for the ionization of weak acid follows:

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

The expression of
`K_a` for above equation:

`K_a=([H^+][A^-])/([HA])`

As,
`[H^+]=[A^-]=4.487* 10^(-3)`

Putting values in above equation, we get:

`8.298* 10^(-5)=((4.487* 10^(-3))* (4.487* 10^(-3)))/([HA])`

`[HA]=0.243M`

Hence. the initial concentration of weak acid solution is 0.243 M