Respuesta :
Answer:
pKa = 4.5
Explanation:
The weak acid can be represented by the general formula, HA and the dissociation equilibrium given as:
[tex]HA \rightleftharpoons H^{+}+A^{-}[/tex]
where HA = protonated form
A- = deprotonated form
The Henderson-Hasselbalch equation relates the pH of a solution to the ratio of the concentrations of HA and A- as;
[tex]pH = pKa + log\frac{[A-]]}{[HA]]}-----(1)[/tex]
It is given that:
% deprotonated i.e. A- = 25%
Therefore, %protonated i.e. HA = 100 -25 = 75%
pH = 4
Based on equation (1)
[tex]4 = pKa + log\frac{[25]]}{[75]]} = pKa-0.477[/tex]
pKa = 4.477 i.e. around 4.5
The pKa of the weak acid is 5.07
Data;
- pH = 4
- α = 25% = 0.25
For Weak Acids
The dissociation constant is given as
[tex]K_a = \frac{c\alpha ^2}{1-\alpha }\\[/tex]
The concentration of the acid can be calculated as
[tex]pH = -log [H^+]\\\\H^+ = 10^-^4\\H^+ = 1*10^-4M[/tex]
substitute the values into the equation above
[tex]Ka = \frac{1.0*10^-^4*0.25^2}{1-0.25}\\Ka = 8.33*10^-^6\\[/tex]
The pKa is calculated as
[tex]pKa = -logKa \\pKa = -log(8.33*10^-6)\\pKa = 5.07[/tex]
From the calculation above, the pKa of the weak acid is 5.07
Learn more on acid dissociation constant here;
https://brainly.com/question/10710178
