Barbituric Acid dissociation constant

[BadatPhysicsguy]

Homework Statement

Barbituric acid is a single proton organic acid. Assume it is called "HA". Results show pH 4.22 in a solution of 0.020mol/dm3 of barbituric acid and 0.030mol/dm3 for sodium barbiturate, NaA. Calculate the "acid dissociation constant" for barbituric acid.

Homework Equations

The Attempt at a Solution

How do I even start? Through the pH I can calculate number of protons (H3O+) but I don't know where to go from here, since they've added "sodium barbiturate" into the whole equation. Would have been easy (I think) if it was just:

HA + H2O ⇔ A- + H30+

But now there is some sodium barbituric acid, where does the sodium come from? Would appreciate help.

 

[Borek]

You can safely assume concentrations of HA and A^- to be these given (in other words: once you put the acid and its salt into solution, their concentrations don't change by much due to dissociation; this is obvious if you know how buffers work).