Question

Sketch a pH curve for the titration of a weak acid (HA) with a strong base (NaOH). List the major species, and explain how you would calculate the pH of the solution at various points, including the halfway point and the equivalence point.

Short answer

$\text{pH}$at half equivalence point is$=\text{pKa}$

$\text{pH}$at equivalence point is$\text{pH}=\frac{\text{1}}{\text{2}}{\text{pK}}_{\text{w}}+\frac{\text{1}}{\text{2}}{\text{pK}}_{\text{a}}+\frac{\text{1}}{\text{2}}\text{logC}$

Step-by-step solution

Definition of titration curve

A graphical representation of the pH of a solution during a titration is defined as titration curve

Graphical representations

The $\text{pH}$curve between the titration of weak acid and strong base

The characteristics of titration curve is

1. The $\text{pH}$is less acidic than the titration of strong acid before the addition of strong base
2. At the start of titration, there is a sharp increase in $\text{pH}$due to anion of weak acid which shows the common ion effect and reduces the ionization of acid
3. After a sharp increase the curve changes gradually due to the solution that acts as buffer. This will continue till the base reduces the ionization of acid.
4. The half neutralization occurs in the middle of the curve. At that point of half neutralization the concentration of weak acid is equal to the concentration of its conjugate base.

At half neutralization point $\mathbf{pH}\mathbf{=}\mathbf{pKa}$

1. $\text{pH}$is greater than at the equivalence point because all acid is converted into its conjugate base by addition of strong base.

$\text{pH}$at equivalence point is $\mathbf{pH}\mathbf{=}\frac{\mathbf{1}}{\mathbf{2}}{\mathbf{pK}}_{\mathbf{w}}\mathbf{+}\frac{\mathbf{1}}{\mathbf{2}}{\mathbf{pK}}_{\mathbf{a}}\mathbf{+}\frac{\mathbf{1}}{\mathbf{2}}\mathbf{logC}$

$\text{C}=\text{\hspace{0.17em}concentration\hspace{0.17em}of\hspace{0.17em}salt\hspace{0.17em}at\hspace{0.17em}equivalence\hspace{0.17em}point}$