Respuesta :
Answer: The initial concentration of weak acid solution is 0.243 M
Explanation:
We are given:
pH of unknown acid at the halfway point of titration = 4.081
We know that:
[tex]pH=pK_a=4.081[/tex] (At halfway point)
To calculate the [tex]K_a[/tex], we use the equation:
[tex]pK_a=-\log (K_a)[/tex]
Putting values in above equation, we get:
[tex]4.081=-\log (K_a)\\\\K_a=10^{-4.081}=8.298\times 10^{-5}[/tex]
As, the initial pH of the acid is 2.348
- To calculate the hydrogen ion concentration, we use the equation:
[tex]pH=-\log [H^+][/tex]
Putting values in above equation, we get:
[tex]2.348=-\log [H^+][/tex]
[tex][H^+]=10^{-2.348}=4.487\times 10^{-3}[/tex]
The chemical equation for the ionization of weak acid follows:
[tex]HA\rightleftharpoons H^++A^-[/tex]
The expression of [tex]K_a[/tex] for above equation:
[tex]K_a=\frac{[H^+][A^-]}{[HA]}[/tex]
As, [tex][H^+]=[A^-]=4.487\times 10^{-3}[/tex]
Putting values in above equation, we get:
[tex]8.298\times 10^{-5}=\frac{(4.487\times 10^{-3})\times (4.487\times 10^{-3})}{[HA]}[/tex]
[tex][HA]=0.243M[/tex]
Hence. the initial concentration of weak acid solution is 0.243 M
