Barbituric Acid dissociation constant

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SUMMARY

The discussion centers on calculating the acid dissociation constant (Ka) for barbituric acid (HA) using the provided pH of 4.22 in a solution containing 0.020 mol/dm³ of barbituric acid and 0.030 mol/dm³ of sodium barbiturate (NaA). The pH allows for the determination of the hydronium ion concentration, which is essential for calculating Ka. The presence of sodium barbiturate, a salt of the acid, contributes to the equilibrium but does not significantly alter the concentrations of HA and A- in the solution. The final calculation of Ka is derived from the equilibrium expression involving these concentrations.

PREREQUISITES
  • Understanding of acid-base equilibrium and dissociation constants
  • Knowledge of pH and hydronium ion concentration calculations
  • Familiarity with buffer solutions and their components
  • Basic algebra for manipulating equilibrium expressions
NEXT STEPS
  • Study the Henderson-Hasselbalch equation for buffer calculations
  • Learn about the relationship between pH and pKa in acid-base chemistry
  • Explore the concept of buffer capacity and its implications in solutions
  • Investigate the dissociation constants of related compounds for comparative analysis
USEFUL FOR

Chemistry students, particularly those studying organic chemistry and acid-base equilibria, as well as educators and professionals involved in analytical chemistry and laboratory work.

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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.

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.
 
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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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