Final answer:
To calculate the pH at different points in the titration, we use the expressions for pOH and pH depending on the concentration of the acid and its conjugate base. Before adding any KOH, the pH is determined by the concentration of the acid. After adding KOH, the pH is calculated using the Henderson-Hasselbalch equation. At the equivalence point and after adding an excess of KOH, the pH is determined by the concentration of the conjugate base and hydroxide ion, respectively.
Step-by-step explanation:
To calculate the pH at different points in the titration, we need to consider the reaction between barbituric acid and KOH. The balanced equation for this reaction is:
HA + OH- → A- + H2O
a) Before any KOH is added, the solution only contains barbituric acid. Therefore, the pH is determined by the concentration of the barbituric acid and can be calculated using the expression:
pOH = -log[HA]
b) After adding 20 mL of KOH, the solution will contain a mixture of barbituric acid and its conjugate base. The pH can be calculated using the Henderson-Hasselbalch equation:
pH = pKa + log([A-]/[HA])
c) At the equivalence point, all the barbituric acid has reacted with KOH to form the conjugate base. Therefore, the pH will be determined by the concentration of the conjugate base and can be calculated using the expression:
pH = -log([A-])
d) After adding 60 mL of KOH, the solution will contain an excess of OH- ions. Therefore, the pH will be basic and can be calculated using the expression:
pOH = -log[OH-]