Question

Alanine has pKa values of 2.34 and 9.69. Therefore, alanine exists predominately as a zwitterion in an aqueous solution with pH > ____ and pH < ____.

Short answer

Alanine exists predominately as a zwitterion in an aqueous solution with pH > 2.34 and pH < 9.69.

Step-by-step solution

The Henderson–Hasselbalch equation

The Henderson-Hasselbalch equation is used to determine the pH of the solution. Whether a given acid loses a proton in an aqueous solution depends on both the pKa of the acid and the pH of the solution. The relationship between the two is given by the Henderson–Hasselbalch equation.

The Henderson–Hasselbalch equation states that:

• When the pH of a solution equals to the pKa of the compound that undergoes dissociation, the concentration of the compound in its acidic form [HA] equals the concentration of the compound in its basic form [A-] (because log 1 = 0).

• When the pH of the solution is less than the pKaof the compound, the compound exists primarily in its acidic form.

When the pH of the solution is greater than the pKa of the compound, the compound exists primarily in its basic form.

Zwitter ion

A zwitterion also known as a dipolar ion, is a compound that has a negative charge on one atom and a positive charge on a non-adjacent atom.

In other words, zwitterion is an inner salt-containing equal number of positively and negatively charged functional groups.

Identify the pH range of the zwitterion of an alanine

Alanine will exist predominately as a zwitterion in an aqueous solution with pH > 2.34 and pH < 9.69