Chapter 15, Problem 15.39P

Interpretation Introduction

Interpretation:

In Section 6.2d, we saw that ${\text{pK}}_a$ of an acid is equal to the $\text{pH}$ of the solution at which half the acid has dissociated into its conjugate base. UV–vis spectroscopy can be used to measure the relative concentrations involved if the acid or conjugate base absorbs UV–vis light. With this in mind, suppose that a particular acid has ${\text{λ}}_{\text{max}}$ of 312 nm. The given figure shows the absorbance at that wavelength as a function of $\text{pH}$. ${\text{pK}}_a$ of the acid is to be determined.

Concept introduction:

If the given absorbance corresponds to the concentration of acid present, half the absorbance shows the acid which is dissociated in the equal concentration of its conjugate base. ${\text{pK}}_a$ of an acid is equal to the $\text{pH}$ of the solution at which half the acid has dissociated into its conjugate base.