Chapter 17, Problem 17.29P

Interpretation Introduction

Interpretation:

It should be explained that the carboxyl group of lactic acid $\left(p{K}_{a}3.08\right)$ can exist whether primarly as a carboxyl group or as a carboxylate anion under the given conditions.

Concept Introduction:

Acid - ionization constant${\text{K}}_a$:

Acids ionize in water.  Strong acids ionize completely whereas weak acids ionize to some limited extent.

The degree to which a weak acid ionizes depends on the concentration of the acid and the equilibrium constant for the ionization.

The ionization of a weak acid $\text{HA}$ can be given as follows,

${\text{HA}}_{(aq)}\rightleftarrows {\text{H}}^{+}{}_{(aq)}+{\text{A}}^{\text{-}}{}_{(aq)}$

The equilibrium expression for the above reaction is given below.

$K_a=\frac{[{\text{H}}^{\text{+}}{\text{][A}}^{\text{-}}\text{]}}{[\text{HA]}}$

Where,

  $K_a$ is acid ionization constant,

  $[{\text{H}}^{\text{+}}\text{]}$  is concentration of hydrogen ion

  $[{\text{A}}^{\text{-}}\text{]}$  is concentration of acid anion

  $[\text{HA]}$ is concentration of the acid

$\begin{array}{l}p{K}_a=-\log K_a\\ p{K}_b=-\log K_b\end{array}$

Smaller value of $p{K}_a$, stronger the acid, smaller the $p{K}_b$, stronger the base.

Equilibrium in all acid – base reactions favours formation of the weaker base or weaker acid.

$\begin{array}{c}pH=-{\log }_{10}\left[H^{+}\right]\\ \left[H^{+}\right]={10}^{-pH}\end{array}$