What is the pKa for the acid HA if a solution of 0.65 M HA and 0.85 M NaA has a pH of 4.75?

Question

~~~I know you find the ph by doing -log of pka; how do you do the reverse when you have molarity? I am completely lost. :-(

Answer 1

You're trying to make this a hard problem when it isn't.
This is a buffered solution, use the Henderson-Hasselbalch equation.
pH = pKa + log (base)/(acid)
They give the pH, base is 0.85 and acid is 0.65. Substitute and solve for pKa.

Answer 2

pKa = 4.63