Final answer:
The pKa of the weak acid can be calculated using the pH at the midpoint of the titration with NaOH.
Step-by-step explanation:
The pH at the midpoint of a weak acid titrated with a 0.100 M solution of NaOH can be used to determine the pKa of the acid. In this case, the pH at the midpoint is 4.2. To calculate the pKa, we can use the equation pKa = pH + log([A-]/[HA]), where [A-] is the concentration of the conjugate base and [HA] is the concentration of the acid. Since the acid is weak, we can assume that it is mostly dissociated at the midpoint, so [A-] is approximately equal to the concentration of NaOH. The concentration of NaOH can be calculated using the volume and molarity of the solution of NaOH. From there, we can substitute the values into the equation and solve for pKa.