The volume at which the pH can be used to determine the pKa of the acid is 7.40 ml if the equivalence point occurred at 14.81 ml of added sodium hydroxide.
The weak acid and the conjugate base will have an equal number of moles at the half-equivalence point as the number of moles of sodium hydroxide will neutralize half of the number of moles of the weak acid which will then produce more conjugate base.
The chemical reaction can be given as;
HA = (H^+) + A^-
Calculation of pKa:
pH = pKa + log[A^-]/[HA]
At half equivalence point, pH = pKa
As, ka = 10^-pka
Therefore, ka = 10^-pH
Volume is given which is 14.81 ml. Therefore, the volume that is used to determine the pKa of the acid can be calculated as follows;
14.81/2 = 7.40 ml
Therefore the volume used to determine the pKa of the acid is calculated to be 7.40 ml
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