Question

Glycine has pKa values of 2.34 and 9.60. Therefore, glycine will exist predominately as a zwitterion in an aqueous solution with the following pH.
A. pH = 2.34 and pH = 9.60
B. pH > 2.34 and pH < 9.60
C. pH < 2.34 and pH > 9.60
D. pH = 2.34 and pH > 9.60

Answer 1

Answer:

Option (B) pH > 2.34 and pH < 9.60.

Explanation:

In a zwitterionic structure, overall negative charge should be 0.

It means, for glycine, the two functional groups e.g. $-NH_{2}$ and -COOH should bear equal and opposite charges in it's zwitterionic structure.

$pK_{a}$ of -COOH (in glycine) = 2.34

Therefore, at a pH > 2.34, -COOH group of glycine exists predominantly as $-COO^{-}$.

$pK_{a}$ of  $-NH_{2}$  (in glycine) = 9.60

Therefore, at a pH < 9.60,  $-NH_{2}$  group of glycine exists predominantly as $-NH_{3}^{+}$.

Hence we can say that at 2.34 < pH < 9.60, glycine exists predominantly in zwitterionic form due to presence of $-COO^{-}$ and $-NH_{3}^{+}$ in equal amount.