Question

A solution of glycine has a ph equal to the pka of glycine. What forms of glycine are present and in what ratio?

Answer 1

The solution of glycine has pH = pKa

Aqueous solutions of aminoacids behave as buffers.

Thus the pH of buffer is calculated using Hendersen equation

pH = pKa + log [conjugate base]/[acid]

and if pH = pKa

llog [conjugate base]/[acid] = 0 or [conjugate acid] = [acid]

That is the acid is 50% dissociated

Glycine is

NH3⁺CH2COOH in its completely protonated form .that is the acid form

its conjugate base form is NH3⁺CH2COO^- the zwitterionic form

Thus these two in equal ratio .

At pH = pKa , [ NH3⁺CH2COOH] = [NH3⁺CH2COO^- ] and these are the two forms present in aqueous solution.