Chapter 17, Problem 94FP

Amino acids contain both an acidic carboxylic acid group (—COOH) and a basic amino group (— ${\text{NH}}_2$ ). The ammo group can be protonated (that is it has an extra proton attached) in a strongly acidic solution. This produces a diprotic acid of the form ${\text{H}}_{\text{2}}{\text{A}}^{\text{+}}$ , as exemplified by the protonated amino acid alanine.

The protonated amino acid has two ionizeble protons that can be titrated with OH-.

For the —COOH group, $p{K}_a=2.34$ ; for the ${\text{-NH}}_2^{+}p{K}_l=9.69$ . Consider the titration of a 0.500 M solution of alanine hydrochloride with 0.500 M NaOH solution. What is the pH of (a) the 0.500 M alanine hydrochlonde; (b) the solution at the first half-neutralization point (c) the solution at the first equivalence point?

The dominant form of alanine present at the first equivalence point is electrically neutral despite the positive charge and negative charge it possesses. The point at which the neutral form is produced is called the isoelectric point. Confirm that the pH at the isoelectric point is

$\text{pH=}\frac{1}{2}\left(p{K}_{at}+p{K}_{aa}\right)$

What is the pH of the solution (d) halfway between the first and second equivalence points? (e) at the second equivalence point?

f. Calculate the pH values of the solutions when the following volumes of the 0.500 M NaOH have been added to 50 mL. of the 0.500 M alanine hydrochloride solution: 10.0 ml_,20.D mL 30.0 mL, 40.0 mL. 50.0 mL, 60.0 mL, 70.0 mL, 80.0 mL. 90 0 mL, 100.0 mL, and 110.0 mL

g. Sketch the titration curve for the 0.500 M solution of alanine hydrochloride, and label significant paints an the curve