What is the percent dissociation of glycine if the solution has a pH = 8.60 and pKa = 9.60?

1 Answer
Nov 22, 2015

It's worth mentioning that Glycine has TWO pKas, not just one. One for the amine portion and one for the carboxylic acid portion. This is called a zwitterion.

The relevant pKa is the #\mathbf(9.60)#, which is of the amine group.

At this pH, glycine is deprotonated on the carboxyl and protonated on the amine group since the pH > pKa1 (~2.2), and pH < pKa2 (9.60). Since the reference pKa is 9.60, we are considering the acid glycine and its conjugate base in which the amine group is deprotonated.

Using the Henderson-Hasselbalch equation, we get:

#pH = pKa + log (([A^(-)])/([HA]))#

#-1.00 = log (([A^(-)])/([HA]))#

#0.100 = ([A^(-)])/([HA])#

Since the number of #"mol"#s of base created is equal to the number of #"mol"#s of acid deprotonated, we can normalize the concentrations to #\mathbf(100%)# and then rewrite this as:

#0.100 = x/(1-x)#

#0.100 - 0.100x = x#

#0.100 = 1.100x#

#0.100/1.100 = x#

#color(blue)(x = 9.09%)#

That means there is #90.91%# left of Glycine as the not-yet-deprotonated form, and there is #9.09%# dissociated.