Barbituric Acid dissociation constant

AI Thread Summary
To calculate the acid dissociation constant (Ka) for barbituric acid, the pH of the solution (4.22) indicates the concentration of hydronium ions (H3O+). The presence of sodium barbiturate (NaA) complicates the calculation, as it introduces the conjugate base (A-) into the equilibrium. However, the concentrations of barbituric acid (HA) and its salt can be assumed to remain constant due to buffer behavior. The dissociation reaction can be represented as HA ⇔ A- + H3O+, allowing for the use of the Henderson-Hasselbalch equation to derive Ka. Ultimately, the calculations will yield the dissociation constant for barbituric acid based on the provided concentrations and pH.
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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.

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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.
 
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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).
 
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