For my lab, I have been asked to calculate the pKa of an unknown amino acid. We titrated the unknown amino acid with NaOH. The pKa needs to be calculated using the Henderson Hasselbach Equation. Obviously you can just get the pH from any point on the graph but what values do you use for the concentration of A and HA?

Question

For my lab, I have been asked to calculate the pKa of an unknown amino acid. We titrated the unknown amino acid with NaOH. The pKa needs to be calculated using the Henderson Hasselbach Equation. Obviously you can just get the pH from any point on the graph but what values do you use for the concentration of A and HA?
I know the concentration of NaOH used (0.5M)
The total volume added of NaOH
mL of NaOH added and equivalents of NaOH added
and I know the unknown amino acid is 10.003 mg/mL. How on earth do you get the concentration values for the henderson hasselbach with these variables?

DrBob222 · Answer

You look at the graph and pick the midpoint (the inflection in the graph). At that point, (HA) = (A^-) and pH = pKa.

Eleanor · Answer

Our professor said we have to use the equation to get the value though, that we shouldn't just pull it from the graph.