1. The problem statement, all variables and given/known data 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. 2. Relevant equations 3. 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.