Question

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

Answer 1

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.